Revised DIIS code to be somewhat simpler and more general.

[lothian]

Description

Modifying DIIS code to be somewhat simpler internally, though the interface will now require the caller to create the error vectors.

Todos

• Modify the DIIS class to use residuals as error vectors and avoid T1- and T2-type tensors as separate arguments.
• Modify the calling codes to create their own error vectors and to 'np.hstack' their amplitudes for extrapolation.

Status

• Ready to go