Refactor integrator API.

examples/src/main/kotlin/space/kscience/kmath/functions/integrate.kt

@@ -5,6 +5,9 @@
 
 package space.kscience.kmath.functions
 
+import space.kscience.kmath.integration.integrate
+import space.kscience.kmath.integration.integrator
+import space.kscience.kmath.integration.value
 import space.kscience.kmath.operations.DoubleField
 import kotlin.math.pow
 
@@ -13,8 +16,8 @@ fun main() {
     val function: UnivariateFunction<Double> = { x -> 3 * x.pow(2) + 2 * x + 1 }
 
     //get the result of the integration
-    val result = DoubleField.integrate(0.0..10.0, function = function)
+    val result = DoubleField.integrator.integrate(0.0..10.0, function = function)
 
     //the value is nullable because in some cases the integration could not succeed
-    println(result.valueOrNull)
+    println(result.value)
 }

examples/src/main/kotlin/space/kscience/kmath/functions/matrixIntegration.kt

@@ -5,6 +5,9 @@
 
 package space.kscience.kmath.functions
 
+import space.kscience.kmath.integration.integrate
+import space.kscience.kmath.integration.integrator
+import space.kscience.kmath.integration.value
 import space.kscience.kmath.nd.StructureND
 import space.kscience.kmath.nd.nd
 import space.kscience.kmath.operations.DoubleField
@@ -22,9 +25,9 @@ fun main(): Unit = DoubleField {
         val function: (Double) -> StructureND<Double> = { x: Double -> 3 * number(x).pow(2) + 2 * diagonal(x) + 1 }
 
         //get the result of the integration
-        val result = integrate(0.0..10.0, function = function)
+        val result = integrator.integrate(0.0..10.0, function = function)
 
         //the value is nullable because in some cases the integration could not succeed
-        println(result.valueOrNull)
+        println(result.value)
     }
 }

kmath-commons/src/test/kotlin/space/kscience/kmath/commons/integration/IntegrationTest.kt

@@ -7,6 +7,7 @@ package space.kscience.kmath.commons.integration
 
 import org.junit.jupiter.api.Test
 import space.kscience.kmath.integration.integrate
+import space.kscience.kmath.integration.value
 import space.kscience.kmath.misc.UnstableKMathAPI
 import space.kscience.kmath.operations.DoubleField.sin
 import kotlin.math.PI
@@ -19,7 +20,7 @@ internal class IntegrationTest {
 
     @Test
     fun simpson() {
-        val res = CMIntegrator.simpson().integrate(0.0..2 * PI, function = function)
+        val res = CMIntegrator.simpson().integrate(0.0..2 * PI, function = function).value
         assertTrue { abs(res) < 1e-3 }
     }
 
@@ -28,7 +29,7 @@ internal class IntegrationTest {
         val res = CMIntegrator.simpson().integrate(0.0..PI, {
             targetRelativeAccuracy = 1e-4
             targetAbsoluteAccuracy = 1e-4
-        }, function)
+        }, function).value
         assertTrue { abs(res - 2) < 1e-3 }
         assertTrue { abs(res - 2) > 1e-12 }
     }

kmath-functions/src/commonMain/kotlin/space/kscience/kmath/integration/GaussIntegrator.kt

@@ -6,7 +6,9 @@ package space.kscience.kmath.integration
 
 import space.kscience.kmath.misc.UnstableKMathAPI
 import space.kscience.kmath.operations.Field
-import space.kscience.kmath.structures.*
+import space.kscience.kmath.structures.Buffer
+import space.kscience.kmath.structures.asBuffer
+import space.kscience.kmath.structures.indices
 
 /**
  * Set of univariate integration ranges. First components correspond to ranges themselves, second components to number of
@@ -81,7 +83,7 @@ public val <T:Any> Field<T>.integrator: GaussIntegrator<T> get() = GaussIntegrat
 
 
 /**
- * Use [integrate] to integrate the function in the current algebra with given [range] and [numPoints]
+ * Integrate using [intervals] segments with Gauss-Legendre rule of [order] order
  */
 @UnstableKMathAPI
 public fun <T : Any> GaussIntegrator<T>.integrate(

kmath-functions/src/commonTest/kotlin/space/kscience/kmath/integration/GaussIntegralTest.kt

@@ -16,7 +16,7 @@ import kotlin.test.assertEquals
 class GaussIntegralTest {
     @Test
     fun gaussSin() {
-        val res = DoubleField.integrate(0.0..2 * PI) { x ->
+        val res = DoubleField.integrator.integrate(0.0..2 * PI) { x ->
             sin(x)
         }
         assertEquals(0.0, res.valueOrNull!!, 1e-2)
@@ -24,7 +24,7 @@ class GaussIntegralTest {
 
     @Test
     fun gaussUniform() {
-        val res = DoubleField.integrate(0.0..100.0) { x ->
+        val res = DoubleField.integrator.integrate(0.0..100.0) { x ->
             if(x in 30.0..50.0){
                 1.0
             } else {