Ternary Option for USE_INTERPOLATION_TABLES

[daviesje]

Currently, there are two kinds of interpolation table controlled by the UserParams field USE_INTERPOLATION_TABLES.

1. the Sigma(M) interpolation table
2. Conditional mass function integral tables.

Since 1. is used to construct 2. and both involve integrals, these become very slow if the flag is set to false, which makes testing the interpolation tables annoying.

We should add a third option to the flag:

• "no-interp" (0 in the C backend): No interpolation tables used at all, for testing the sigma tables on limited runs
• "sigma-only" (1 in the C backend): Use interpolation tables for sigma, but not the CMF, used for testing the CMF tables and/or slower but more accurate runs
• "all-interp" (2 in the C backend): Use all interpolation tables.

While this change should be easy to make, we should wait for PRS #415 and #427 to avoid clashing, since this will change lines all over the place in the backend and frontend.