WIP #1034 NSum, NExpectation NProbability

- use interfaces `IDiscreteDistribution, IContinuousDistribution`
- defined keyword `NProduct`

symja_android_library/doc/functions/NSum.md

@@ -0,0 +1,24 @@
+## NSum
+
+```
+NSum(expr, {i, imin, imax})
+```
+
+> evaluates the numerical approximated sum of `expr` with `i` ranging from `imin` to `imax`.
+      
+```
+NSum(expr, {i, imin, imax, di})
+```
+
+> `i` ranges from `imin` to `imax` in steps `di`
+
+See
+* [Wikipedia - Summation](https://en.wikipedia.org/wiki/Summation)
+* [Wikipedia - Convergence_tests](https://en.wikipedia.org/wiki/Convergence_tests)
+
+### Examples
+
+```
+>> Num(k, {k, 1, 10})    
+55    
+```

symja_android_library/doc/functions/Sum.md

@@ -27,6 +27,7 @@ Sum(expr, {i, imin, imax}, {j, jmin, jmax}, ...)
 		
 See
 * [Wikipedia - Summation](https://en.wikipedia.org/wiki/Summation)
+* [Wikipedia - Faulhaber's formula](https://en.wikipedia.org/wiki/Faulhaber%27s_formula)
 
 ### Examples
 

symja_android_library/matheclipse-core/src/main/java/org/matheclipse/core/builtin/FunctionDefinitions.java

@@ -93,6 +93,7 @@ private static void init() {
       S.NonCommutativeMultiply
           .setEvaluator(new org.matheclipse.core.reflection.system.NonCommutativeMultiply());
       S.NSolve.setEvaluator(new org.matheclipse.core.reflection.system.NSolve());
+      S.NSum.setEvaluator(new org.matheclipse.core.reflection.system.NSum());
       S.NumberLinePlot.setEvaluator(new org.matheclipse.core.reflection.system.NumberLinePlot());
 
       S.Out.setEvaluator(new org.matheclipse.core.reflection.system.Out());