Get rid of proliferation of foo_transpose functions in PETScLUSolver

[Prof Garth Wells]

These are really ugly and unnecessary, and they're not available in PETScKrylovSolver. Plus the solve functions have a bool for transpose. How did it become such a mess?

Better solution is optional enum to solve functions.

[Prof Garth Wells]

Fixed in f6299d2.

This mess went a lot deeper, but has now been cleaned out.