Multiple same-size linear solver

Solve multiple systems of linear equations of the same size

Sometime we need to solve a bunch of small linear (least-square) systems of the same size. This function can accomplish this task conveniently.

The system matrix and RHS are passed as 3D arrays.

Furthermore, calling syntax can support two simplifications:
- same RHS for all systems; or
- single RHS for each system.

Followed an idea from Tim Davis (using sparse)