Cleanup LaplaceFEM solver

[rrsettgast]

The LaplaceFEM solver needs to be cleaned up and documented. It will serve as the template for developing new solvers via the Finite Element Method.

[joshua-white]

Also need to remove XML finite element input as a requirement for all solvers.

[joshua-white]

Need to consider more general FEM design issues, like separation of mesh from support points, enabling higher order elements, and mixed finite element spaces.