Deal with non-orthogonal basis

MeanFi currently only deals with an orthogonal basis. Implementing a solver that handles non-orthogonal basis would require including overlap matrices in the algorithm in the following steps:

• constructing the tight-binding model,
• calculating the Hartree-Fock-BdG corrections,
• using a generalized eigensolver,
• and computing density matrix.

It is doable since quantum chemistry has done it for decades.