Implementation of discrete summations

[emanbau]

Implemented a function and some unit tests to handle discrete summations (Σ) given a user provided function f(n), start, and end integers as inspired by #3075. Hopefully this acts as a good start for this functionality!

[josdejong]

Thanks Eman, this is a nice start indeed! Thanks for picking this up.

What functionality do you think this first implementation should have? I think we should make the ideas discussed in #3075 concrete. Some thoughts from my side:

1. implement both sum and prod
2. support not just number but also other data types
3. decide whether we want to create this as new, separate functions, or extend the existing sum and prod
4. decide on the order of arguments (I think I'm ok with sum(start, end, fn) but would like to check what other math application do)
5. implement support for range?