Mesh:
  Add generic interface functions
    - integral_prod_on_ref_fe_side_vs_outward_normal, to allow integrating polynomial * monomial vector against an outward normal on a face, and to allow mesh implementations to specialize products against outward normals on faces efficently (avoid allocation if trivially zero).
    - function num_side_faces_per_fe: to allow clients to enumerate sides, e.g. to precompute integrals over faces
    - space_dim
  Renamed Face to FEFace for clarity and introduced constructor function fe_face.

WGBasis
  - Add interior/side_monomial_num and accessors interior/side_monomial_by_num to allow clients to create caches of computations by monomial number.
  - Add functions num_interior/side_monomials_per_fe
WGMethod
  - Created WGSolver type to hold precomputed values not depending on rhs function f in the variational problem or boundary value function g.
  - Created basis tests file.