Limited Memory Broyden Rank Reduction

See https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.broyden2.html#scipy-optimize-broyden2 and https://web.archive.org/web/20161022015821/http://www.math.leidenuniv.nl/scripties/Rotten.pdf for rank-reduction.

The currently implemented version corresponds to "simple". We would want to have the other variants as well.