Add external C code for reading new format CSO fit data files

These files were developed outside of SMURF for reading CSO fit data
files. The data files use the new format developed for SCUBA-2 rather
than the old format used for SCUBA and defined in the JCMT::Tau::CsoFit
perl module. Hence the use of the "2" in the names.

An example fit data file is included covering 20130313 to 20130501.
smurf: Csofit parser should use timegm rather than mktime

mktime is for local time.
smurf: Add TAI to unix epoch conversion

The CSO fit library routines require unix epoch seconds but SMURF
uses TAI MJD internally.

This routine includes an internal sanity check by round-tripping the epoch
back to TAI and comparing with the originally supplied value. This can be
removed when we believe the routine is working properly.
smurf: Add routine to calculate CSO fit

Initial implementation of routine for calculating the tau from CSO
fits. It's a little inefficient because it opens the entire fit
file every time and does not attempt to cache the result.

Also enables the building of the csofit code.
smurf: Add CSOFIT tau source

- Does not support AUTO fallback from WVM to CSO.

- Very inefficient because the fit parameters are relcalculated for
each file by re-reading all the fits and then subsetting the fits
for the file. Ideally we should read the fits once and calculate
subsets for each of the relevant UT dates and pass those subsets into
the extinction routine.