Tidy pre processor options

[simonrp84]

The ORAC pre-processor contains multiple options, set through the parse_optional subroutine (utils_for_main.F90) based on what's in the driver file. This means we're passing huge numbers of variables between options, which is untidy.
An 'optional variables' struct would tidy this up and make the code more readable.

[adamcpovey]

The code that generates the parser for the main processor was designed to be general and create multiple parsers. We should probably tidy the preprocessor and putting all the options in an equivalent to the Ctrl structure will help make a better parser.

[simonrp84]

I'm working on this now, have created a "preproc_opts" type that contains all the option variables (plus ecmwf filenames, etc).