Add possibility to generate DG-DG and DG-Lagrange, and DG-P2PlusBubble forms

Notes:
- The functionality is required for issue #49.
- Generation of DG-DG operators currently seems to be a more theoretical
  perspective. See issue #19.

Status:

- The high level part, such as form definition and name generation works.
- We can successfully generate fro triangles the following forms:
  * dg1_to_p2_plus_bubble_divt_affine_q3
  * dg1_to_p2_plus_bubble_divt_blending_q3
  * p2_plus_bubble_to_dg1_div_affine_q3
  * p2_plus_bubble_to_dg1_div_blending_q3