Remove mesh.compute_boundaries

It's always been a no-op and we're never going to use it.
io: Ensure output directory exists
Correct convergence orders in nonlinear helmholtz test
Support projection of UFL expressions
Use solver_parameters not parameters in NLVS

To be consistent with solve, use solver_parameters to specify parameters
to the solver.
Mention MixedVectorSpaceBasis in solve docs
Merge pull request #282 from firedrakeproject/minor_fixes

- Mention MixedVectorSpaceBasis in solve docs
- Use solver_parameters not parameters in NLVS
- Support projection of UFL expressions
- Correct convergence orders in nonlinear helmholtz test
- io: Ensure output directory exists
- Remove mesh.compute_boundaries