Sum overload

Overloaded the sum function to allow spot operators as input. Also added wrappers
for CGLS and LSMR least squares solvers. LSMR can be downloaded here http://www.stanford.edu/group/SOL/software/lsmr.html. The CGLS routine from http://www.stanford.edu/group/SOL/software/cgls.html, does not have the usual syntax CGLS(A,b,.. ). Therefore I've updated this routine and cleaned up the documentation. If you would like to have it I can also include it.