Refactor ngrid

[marco-2023]

Refactored the ngrid module to improve readability and add features.

• Added weights property which returns the weights for each argument combination (due to its combinatorial nature returns a generator)
• Added points property which returns the combinations of arguments (due to its combinatorial nature returns a generator). Each point is a tuple with several arrays, each array corresponds to an argument of the multigrid function to integrate.
• Added support for non-vectorized functions. These will be evaluated point by point during the integration.

@PaulWAyers I think this PR improved the previous class. Nonetheless, I still have some doubts:

1. The current class used is a derived class of Grid because of this, several methods are inherited moments, save and get_localgrid. I think that save and get_localgrid can be implemented (I have some doubts about the meaning of get_localgrid in this context ), but I don't see how to get moments.
2. This one is related to my previous doubt. Wouldn't be better to make a new class called perhaps MultiDomainGridIntegrator that is only responsible for integrating functions with more than one argument?

[PaulWAyers]

Thanks @marco-2023 . I'm going to add @FarnazH to the review panel for this one. I'm also curious if @Ali-Tehrani has any comments/thoughts.

[PaulWAyers]

I talked to @marco-2023 about this today. The main issue is that the basegrid defines moments and local grids. Doing local grid is "easy" for ngrid (Marco says it's ~30 minutes of work.)

I looked a little bit at the code. Some comments:

1. The moments seem tied to 3-dimensional integration. I don't think it is worth trying to define n-dimensional spherical harmonics. (They exist, but it seems more painful than it is worth, but it is feasible.) This would make it possible to define "all the types of moments". I do not know a use case for this, however.

2. I think the easiest solution might be to support only Cartesian moments. The order of a Cartesian moment in ngrid is just the sum of the orders of the Cartesian moments of the composing low-dimensional grids.

What do you think @Ali-Tehrani and @FarnazH ?