Add SolverGCR

[peterrum]

I am working on this together with @dravinsi.

[tjhei]

Do you have a good reference for the algorithm?

[dravinsi]

Do you have a good reference for the algorithm?

I apologize for the late reply. The original GCR is mentioned the GMRES paper by Saad and Schultz, found here https://web.stanford.edu/class/cme324/saad-schultz.pdf

A preconditioned variant of GCR (referred to as GMRESR) is described in a paper by H. A. Van der Vorst & C. Vuik, found here https://onlinelibrary.wiley.com/doi/abs/10.1002/nla.1680010404

The latter reference was obtained from Y. Notay's paper on AMG found here https://etna.math.kent.edu/vol.37.2010/pp123-146.dir/pp123-146.pdf where it is reference [40]. This paper also includes a compact pseudocode.

I will also stress what Peter has said; that this is a work in progress.

[bangerth]

@dravinsi When you write the documentation, please add these references to the file doc/doxygen/references.bib and then cite them via @cite TAG. This way, people can jump from the class documentation to the paper in question.

An easy way to generate a bibtex tag for doc/doxygen/references.bib is to use https://doi2bib.org .