Reduction To Tridiagonal "1-stage" (local)

[albestro]

This effort aims at evaluating a possible improvement for the GPU backend by using a 1-stage reduction instead of the 2-stage currently implemented which seems to fit better on a MC backend.

At the moment of writing:

• a basic local MC implementation (not refined, just "make it work")
• a basic test
• a trivial swap of the algorithm in eigensolver from 2-stage to 1-stage, since there no miniapp has been implemented yet

Next steps:

• making it work on GPU backend
• profiling and analysis for identification of main problems for better defining optimization strategies