Feature/np cumsum

[Plankton555]

Simple implementation of cumulative sum. Either call as a member function from an NDArray or as a static function via np.cumsum().

[Nucs]

Looking good, just missing some documentations.

[Nucs]

Add documentation similar to this

[henon]

Please always add the tests from the numpy docs examples too:

https://docs.scipy.org/doc/numpy-1.15.1/reference/generated/numpy.cumsum.html

[Oceania2018]

@Plankton555 Glad you get started in NumSharp jounery. Just a little suggestion about this PR.
Try this Assert.IsTrue(Enumerable.SequenceEqual(arr, expected)); instead of for loop.

[Plankton555]

@Oceania2018 I tried using Linq instead but didn't get it to work. Any advice are welcome

Assert.IsTrue(Enumerable.SequenceEqual(arr, expected));
resulted in
The type arguments for method 'Enumerable.SequenceEqual<TSource>(IEnumerable<TSource>, IEnumerable<TSource>)' cannot be inferred from the usage. Try specifying the type arguments explicitly

Assert.IsTrue(Enumerable.SequenceEqual<double>(arr, expected));
resulted in
cannot convert from 'NumSharp.NDArray' to 'System.Collections.Generic.IEnumerable<double>

[Oceania2018]

Assert.IsTrue(Enumerable.SequenceEqual(arr.GetData<double>(), expected.GetData<double>()))

[Plankton555]

• Tests from the examples in the numpy documentation have been added.
• Added documentation to functions.

4/6 tests are failing currently. The failing tests are related to summing over specific axes, as well as specifying data types.

[Nucs]

Thank you for the PR!