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
Merge pull request #61 from grinsfem/ic_values

Ic values