[utils] Identity*Identity = Identity ;)
[utils] rewrite norm + inner

[skip ci]
[utils] include preconditioning in Arnoldi

[skip ci]
[utils] Arnoldi/Lanczos fixes
[linsys] new Minres class
[linsys] allow Minres with other orthogonalizations

[skip ci]
[linsys,tests] let Minres pass tests
[linsys] fix minres
[utils,test] adapt to norm/inner changes
[linsys] introduct _Solver._prepare()
[tests] minor pep8 change
[linsys] new Gmres class
[linsys, tests] adapt Gmres tests
[tests] pep8
[utils] Arnoldi/Lanczos handle imaginary values
[linsys] comment on real valued R in Minres
[tests] remove csr from testset
[tests] sanitize linsys tests
[linsys] fix some harmless complex/real warnings
[tests] fix Gmres tests