Add inference functions for 2D

[TomMaullin]

The following 3D functions should have 2D counterparts:

• [x] get_resms3D
• [x] covB3D
• [x] varLB3D
• [x] get_T3D
• [x] get_F3D
• [x] get_R23D
• [x] T2P3D
• [x] F2P3D
• [x] get_swdf_T3D
• [x] get_swdf_F3D
• [x] get_dS23D
• [x] get_InfoMat3D

And corresponding tests.

[TomMaullin]

This issue has now been addressed by issue #35.