Meanfield module refactoring

[matt-chan]

The meanfield module should be slightly refactored to make picking and choosing easier.

The SCF and abstract Hamiltonian should not have any references to HORTON datatypes. The HF and DFT hamiltonians should be separated into their own files, and can make reference to HORTON datatypes such as gbasis or grid. That way, an external user can choose to take the meanfield module alone and delete the HF and DFT modules if they want to break the dependencies on HORTON.

[tovrstra]

This is more-or-less already done. The only thing that needs to be changed is to remove the imports from horton.meanfield.__init__.py.