Add support for bc_variables, bc_values config

And add a basic "dirichlet" bc_types option which sets a single
variable to a value given by a parsed function.

I don't think I like this interface yet, but we can change it to
something nicer later.
Starting a "convection_cell" test

Right now we're just forking lid_driven_cavity; we'll change that
shortly to incorporate a dirichlet ParsedFunction
dirichlet gets "constant_" and "parsed_" versions
Merge pull request #60 from grinsfem/bc_values

Boundary Condition values