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
domain_data -> subdomain_data
update ufl Argument counts from -2, -1 to 0, 1
domain_type -> integral_type
domain_id -> subdomain_id
integral.measure() deprecation changes
Add ufl_domain and ufl_id to Mesh
Construct FacetNormal on domain not cell
When building finite elements, use ufl_domain not ufl_cell.
Coordinates live in domain().data().coordinates
measure().integral_type() -> integral_type()
No more integral.measure()
argument .count() -> .number()