added support for expm for using dense linear solver, and defaulting to using dense solver for small/dense problems and the sparse solver only for large and sparse problems
added unit tests for sparse/dense expm
require both large and sparse ssystem for using the sparse linear solver in sp_expm
merge master
fine tuned heuristic decision between sparse and dense expm
Merge pull request #182 from jrjohansson/qobj-expm-optionally-dense

added support for expm for using dense linear solver