add tests for spsolve
add support for ndarray inputs in spsolve
FIX: spsolve deals with vectors improperly
sparse: clean up spsolve code a bit + fix issues with vectors
Merge pull request #3061 from pv/pr-3061

BUG: sparse.linalg: fix bugs in how spsolve deals with "vectors" and ndarray input

This preserves the previous semantics of squeezing (N, 1) input to (N,)
dense output.