restore 'iterative-bicg' and 'iterative-bicgstab' as possible interative solvers (they are still included in the docstrings, so they must either be restored or removed from the docstrings if there is no reason to support them)
update docstring
Merge pull request #214 from jrjohansson/restore-bicg-bicgstab

restore 'iterative-bicg' and 'iterative-bicgstab' as possible iterative solvers