ENH: don't solve linear system twice in a row

This is an extremely minor change.

On line 120 we write `self.F = solve(S1, S2)`. Then on line 122 we call `solve(S1, S2)` again. This change just reuses the updated value of `self.F` instead of recomputing. 

All the other changes that show up in the diff are just my editor removing trailing whitespace from lines.