Extend ADMM to general case with A and B

[mrava87]

Currently the ADMM solver accommodates only for the special case of A and B identities.

We should extend the solver to more general cases and have A=None and B=None as defaults (which means identities)

[mrava87]

@NickLuiken here we can make a new ADMML1 with our L1-L1 solver