[PL] Add mesh node ids for BoundaryCondition.
[PL] Initialize node ids vec in boundary conditions.
[PL] GW: Add application of the DirichletBC.
[ML] Move applyKnownSolution() includes in single file.
[PL] Make BoundaryConditions private vars protected.

For usage in derived classes.
[PL] Move findMeshNodes() into UniformDirichletBC.

This function is applicable only to Dirichlet boundary
conditions and not to the Neumann.

Remove default searching of the nodes for _all_ boundary conditions.
[PL] Add boundary condition iterators in ProcessVariable.

Add loop in GW Process in initialize().
[PL] Add ProcessVariable::getMesh().
[PL] Add initialize() to boundary conditions.
[MGTL] Add memoization for MeshNodeSearcher on Mesh ids.