Add subroutines and reduce code duplication [sub-daily CDM lite]

[rjhd2]

Lots of duplicated code. Handle each variable in a separate subroutine, and then pull out identical code snippets where possible into further subroutines.

Preparation for using look-up dictionaries etc for some of the conversions.

[rjhd2]

Next bit is the QC tables codes, and then another go through to find any last bits that could be combined.

[rjhd2]

Have added a number of functions to process the dataframes. Much shorter code. Futher shortening could be done by using a loop across all variables, but this has not been done so far - in order to keep legibility for the coders. Will be addressed in #22