[REQUEST] Support interface to C++ integral libraries

[wilhadams]

Is your feature request related to a problem? Please describe. For standard integrals, it would be beneficial to support fast C++ libraries, which will likely remain faster than gbasis regardless of any optimizations we make.

Additional context

There are many possible libraries: libint2, libcint, libmint, libreta, ...

Psi4Numpy uses libmints to access the libint library

[wilhadams]

PySCF uses libcint as their AO integral engine

[PaulWAyers]

@msricher has a detailed list of things to think about for this (via e-mail), which she should add to the issue for discussion.

The other thing I know to consider is libreta.

Both libint and libcint seem to be actively maintained on GitHub, which is a point in their favor.

I'll let @msricher put her comments here, and then we can make a decision on the best interface.

[PaulWAyers]

Not urgent, but the most useful "additional feature" we could have would be an interface to the derivative integrals we need for geometry optimization. We haven't supported that in the past, but we could do it easily with automatic differentiation too.

[PaulWAyers]

We considered libint2, libcint, libmint, and libreta and decided to support libint. (Hat tip to @msricher for guiding this decision-making process.) The basic rationale for libcint was that it was stand-alone (unlike libmint) and that its shared object (.so file) can be directly loaded into Python; it was also the package that @msricher was most familiar with. We don't intend to support other libraries, but would welcome contributions from others along those lines.

This was closed with pull request #137