Begin implementation of PeriodicBoundaryCondition construction from PeriodicBoundaryPairs

Still need to work on special boundary types (e.g. sliding interfacing and periodic boundaries)

Expanded on magnetostatic testing with a uniform current density on a circular domain

Nodes, Triangles, Regions are complete (except material properties). Still need to work on Boundaries.

Definition of static constexpr double[] in TriangleQuadrature structs were behaving differently between different versions of gcc. The latest gcc 6.3.1-1 didn't like the fact that the ODR was being violated, whereas some previous versions didn't care.

The definitions need to be visible in the Physics headers, so the quadrature rules were moved to a separate library and the definitions were moved to a cpp file.

Upgrades test of relative rotation motion using a SlidingInterface to include spatially varying excitation so that the motion is observable

Initial implementation and testing of relative motion with a SlidingInterface

Initial implementation and testing of periodic/antiperiodic boundary conditions for magnetostatic solvers

More infrastructure work for periodic b.c.s

Simplification and clarification of (discrete) boundary conditions effecting the implementation of FiniteElementMesh classes

Framework for multi-material magnetic models and testing of linear material properties with relative permeability > 1

Infrastructure and planning for multi-material models

Started implementation of Magnetostatic solver
Work on Dirichlet boundary conditions (magnetic insulation)

Work on HomogeneousForcing for Poisson-like equations
Started implementation of Magnetostatic solver

Work on HomogeneousForcing for Poisson-like equations
Started implementation of Physics template interface

Work on HomogeneousForcing for Poisson-like equations
Started implementation of Physics template interface

Refactor tests and matrix construction to utilize interfaces for XMatrixGroup classes