IntegralHelper -- Generating Integrals

[gustavojra]

Currently, integrals are obtained from an integral helper object by using a dictionary-like call

ints["S"]

which involves the getindex function.

I don't love the current system because it relies on several if statements... It is verbose and hard to extent. I want a way to leverage the multiple-dispatch system such that others can easily implement their own integral helper subtypes.

Thus, we should navigate away from

ints["S"]

and start using a functional form, such as

overlap(ints)