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
First skeleton of initial conditions handling code
Assorted fixes

ICs don't work yet, but "make" and "make check" do.
Construct and properly attach subfunctions
Default to 0 for incomplete CompositeFunctions

I'm not sure this is the right API.  Return a special code which
libMesh can treat as "leave existing values unchanged"?  Throw an
error, but trust to the MultiphysicsSystem to test for and fill in
missing indices with a ZeroFunction?
Enable a GenericICHandler for all physics
Bugfix for string-to-T conversions
System::init_data must precede project_solution

Also, we can project_solution with an incomplete CompositeFunction;
the unset variables just get left as 0
Get one-variable-per-IC working for now

We need better string parsing to handle lists of variables
dirichlet gets "constant_" and "parsed_" versions
Merge pull request #60 from grinsfem/bc_values

Boundary Condition values
Merge pull request #61 from grinsfem/ic_values

Ic values
Bugfix I forgot to push earlier
Missing include fix hadn't been pushed either
Bump up tolerance 60%

I got a ~5.2e-12 level error here on stampede's login node with 1 core.
Merge pull request #62 from grinsfem/bump_tol

Bump up tolerance 60%
Overriding build_context() in preparation for subclassing FEMContext.
make check passing.
Adding AssemblyContext subclass of FEMContext. Not being used
yet.
Have build_context create AssemblyContext instead of FEMContext.