Add option to always include certain bitstrings in diagonalization subspace

[kevinsung]

What should we add?

Sometimes you might want to ensure that certain bitstrings, like the Hartree-Fock bitstring, are always included in the subspace.

[caleb-johnson]

Would this be something used in the helper functions in the counts module?

[kevinsung]

My initial thought was to let the user pass the bitstrings as an argument to solve_fermion.