Implement general euclidean space rectangular mesh in module RMesh with unit tests.
Remove boundary side count from general mesh interface.
Correct integral_on_ref_fe_face method for constants.
For RMesh2/3:
  implement is_boundary_side,
  change mesh coordinates type name,
  introduce dimensions arrays to hold the fe dimensions and for Poly integral calls,
  implement integral_prod_on_fe_face
  add unit tests for integral all methods
Change Poly integral and mon/polynomial value function signatures to use arrays explicitly to avoid allocation of temporary arrays.  Change callers to pass pre-allocated arrays.
Add binary minus operator for polynomials and monomials.
Add unit tests for projections module and fix side projection method signature error when retrieving basis L2 ips matrix.
Note: The side basis elements are not correct, will be fixed subsequently.  See notes for details.
Mesh:
  Add functions dependent_dim_for_nb_side, dependent_dim_for_ref_side_face.
Poly:
  Add function monomials_of_degree_le_with_const_dim.
  Reimplemented monomials_of_degree_le with greater efficiency and with support for domain dimensions up to 5.