Dirichlet boundary condition for GroundwaterFlowProcess.

This adds initialization of boundary conditions (for now only the Dirichlet type) in groundwater flow process. There are some new operators in the ProcessVariable class: getMesh() and boundary condition iterator access. The MeshNodeSearcher class has a new method getMeshNodeSearcher(mesh) which creates a new MeshNodeSearcher for the given mesh and stores it in a private static variable, or returns an existing one.