Sparse matrix linear solver added to GSL

[Patrick Alken <patrick dot alken at Colorado dot EDU>]

Hello all,

  The sparse matrix routines are in pretty good shape now and they are 
all located in a branch called 'sparse' on the gsl git. Furthermore I've 
added a sparse linear solver (for A x = b) based on the Generalized 
Residual Minimum Method (GMRES) iterative solver. GMRES is considered a 
very good/robust solver for a wide class of matrices, though it does 
have convergence issues on some matrices. In these cases it helps to 
precondition the system but I have not currently implemented any 
preconditioners. Maybe one day I'll get around to that.

Everything is documented, with an example of solving the 1D Poisson 
equation sparse system, and I've added lots of automated tests so things 
are working well. Anyone who uses sparse matrices in their work I'd be 
interested in any feedback. I plan to merge the sparse branch into the 
master branch in the next week or so.

Patrick