Add Flatten to monads

[csillikd-messerli]

I deliberately chose Flatten; I find Join less self-explanatory.

[tautropfli]

I haven't really had a need for Flatten, but I also don't have any strong preference against it. Maybe @FreeApophis or @Mafii have a preference for/against this?

One thing to note is that for WhereSelect we chose to go with an overload for flattening:
#448 I'm not sure if SelectMany() is very clear though, so Flatten is probably the better choice?

[tautropfli]

code lgtm otherwise

[FreeApophis]

I think Flatten is good choice as a name, but I think we want to rename WhereSelect too in that case.

review addressed