Add support for optional analytic approximation calculator argument to grids

[marco-2023]

This parameter should be a callable with an analytical integration method. The promolecilar trics using bfit should be a specific case. See #310 for the general idea. This parameter should be optional (for preserving backward compatibility), and the behaviour of the grid methods when it's not present should be the current one.

[Arun-sunny23]

"Hi @marco-2023, I'd like to work on this issue! Could you please assign it to me?"

[PaulWAyers]

@Arun-sunny23 thanks for your interest.

This is an interesting issue, and we can try to sketch out the details before implementing. When implementing, we'll need to be sure that it is done at a time where the maintainers have some spare capacity, or otherwise PRs tend to languish. I'm not sure what @marco-2023 and @FarnazH have going on right now, however.