Adding VERA Core Physics Benchmark Problem 2 two-dimensional examples with CMFD

[wdhawkins]

Below is a comparison of PI vs. PI+CMFD for these problems. Average speedup across the entire suite is ~3X.

| case | ranks | pi_time | pi_sweeps | pi_k      | cmfd_time | cmfd_sweeps | cmfd_k    | pcm diff | speedup | pi_wall_time | cmfd_wall_time |
|------|-------|---------|-----------|-----------|-----------|-------------|-----------|----------|---------|--------------|----------------|
| 2A   | 64    | 574.2   | 181       | 1.180445  | 206.8     | 68          | 1.1804473 | -0.23    | 2.78    | 579.3        | 210.0          |
| 2B   | 64    | 547.2   | 175       | 1.1810156 | 234.6     | 64          | 1.1810164 | -0.08    | 2.33    | 552.4        | 238.1          |
| 2C   | 64    | 557.9   | 175       | 1.1716567 | 209.8     | 64          | 1.1716575 | -0.08    | 2.66    | 563.0        | 213.1          |
| 2D   | 64    | 529.2   | 176       | 1.1644784 | 195.1     | 64          | 1.1644789 | -0.05    | 2.71    | 534.1        | 198.4          |
| 2E   | 64    | 598.6   | 198       | 1.0674144 | 203.3     | 64          | 1.0674144 |  0.00    | 2.94    | 603.8        | 206.6          |
| 2F   | 64    | 722.3   | 236       | 0.973122  | 191.6     | 64          | 0.9731222 | -0.02    | 3.77    | 727.4        | 195.0          |
| 2G   | 64    | 773.4   | 256       | 0.8444559 | 171.9     | 60          | 0.8444565 | -0.06    | 4.50    | 778.4        | 175.2          |
| 2H   | 64    | 794.0   | 260       | 0.7846107 | 189.9     | 64          | 0.784611  | -0.03    | 4.18    | 798.9        | 193.2          |
| 2I   | 64    | 570.7   | 182       | 1.1787303 | 210.8     | 64          | 1.1787311 | -0.08    | 2.71    | 575.6        | 214.1          |
| 2J   | 64    | 722.6   | 236       | 0.972526  | 272.6     | 88          | 0.9725258 |  0.02    | 2.65    | 725.8        | 275.8          |
| 2K   | 64    | 698.9   | 231       | 1.0171608 | 200.9     | 64          | 1.0171609 | -0.01    | 3.48    | 703.9        | 204.1          |
| 2L   | 64    | 533.6   | 162       | 1.0166357 | 196.0     | 60          | 1.0166366 | -0.09    | 2.72    | 538.7        | 209.3          |
| 2M   | 64    | 476.1   | 148       | 0.9365376 | 200.9     | 60          | 0.9365387 | -0.11    | 2.37    | 481.3        | 204.2          |
| 2N   | 64    | 757.8   | 237       | 0.8660509 | 206.5     | 60          | 0.8660514 | -0.05    | 3.67    | 763.0        | 209.8          |
| 2O   | 64    | 531.0   | 167       | 1.0463823 | 291.8     | 92          | 1.0463824 | -0.01    | 1.82    | 536.0        | 295.0          |
| 2P   | 64    | 562.0   | 183       | 0.9264445 | 217.8     | 68          | 0.9264448 | -0.03    | 2.58    | 567.0        | 221.0          |
| 2Q   | 64    | 1233.3  | 415       | 1.1667428 | 268.1     | 92          | 1.1667426 |  0.02    | 4.60    | 1238.1       | 271.3          |