Eigen refactor

[ryanmrichard]

Is this pull request associated with an issue(s)?
No.

Description
allocator::Eigen and buffer::Eigen being templated on the rank is causing problems in contractions. Realistically, allocator::Eigen should be able to allocate any buffer::Eigen based on RUNTIME information. This PR type erases the rank from the public API to accomplish this (the PIMPL is only templated on the floating point type).

Note this PR is going to be API breaking to anything that was directly touching Eigen tensors cough SCF and Integrals cough.

TODOs
I think I have moved all the functions to source that need moved. I still need to:

• Add new methods to allocator::Eigen to support initializing the buffer.
• Add new methods to allocator::Replicated to support getting elements.
• Rewrite tests/input infrastructure that directly touched Eigen tensors.
• Get whatever is still broken to compile.
• Add tests for new functions.

I do not intend to fix the contraction issue @jwaldrop107 found in this PR (this PR is big enough already). I'll tackle that in the next PR.

[ryanmrichard]

@jwaldrop107 this is r2g.