Document content of stats

[gdalle]

It would be nice to have more details on the meaning of each field in the Stats types. Typically I struggle to interpret the difference between solved and inconsistent.

[amontoison]

Yes, I wanted to that since a long time.
inconsistent means that the right-hand side b is not in the range of A. Ax = b can't be solved.
However you can still solve min ||b - Ax|| if the solver has stopping conditions on ||r_k|| and ||A'r_k||.

[amontoison]

#917 provides a documentation for the most relevant structure SimpleStats.
I still need to document the other ones.