FBCache design considerations

[alphaville]

Why do we store m_res1_rows and m_res1_cols as private fields in FBCache? Since we store m_L1 we can retrieve them from there any time we like. The same holds for m_x (we store m_x_rows and m_x_cols).

Further, we don't even need to store m_L1, m_f1, etc. This is why you introduced FBProblem as a field in FBCache. We have the constructor FBCache(FBProblem & p, Matrix & x, double gamma); so we can retrieve a reference to an FBProblem and store it internally and this will be it.

I believe we can/should refactor FBCache and simplify it.

[alphaville]

Done in 59cd77c, but:

1. there are still a few issues such as FBCache unnecessary allocation #21
2. the documentation is not complete
3. I believe m_status < FBCache::STATUS_FORWARD is not very safe
4. A good part of FBCache is not tested