House keeping

[pcubillos]

Minor bug fixes:

• [x] pass nproc from PB to MC3.
• [x] warning when wavenumber sampling does not match EC table wn sampling.
• [x] Skip path check when mode=opacity.
• [x] skip tmin, tmax check if extfile exists.
• [x] skip bulk check if retflag does not have mol flag.
• [x] Use backslash for long line commands in documentation (for pdf).
• [x] Remove checks for rpars, hpars, and cpars when parameters are in MCMC params input.
• [x] Remove system parameters check for opacity run.