OK chaps @bouweandela @zklaus @ledm here is the long promised implementation of fx vars handling: this is a sister PR of #21 but with much improved user interface and functionality. It does the following:

• allows for use of fx variables (CMIP5 and 6) as regular variables;
• removes any special file retrieval functionality for fx files; the regulat get_input_filelist is used; also removes the special entries for fx files/dirs in config-developer;
• removes the need for the user to specify the fx_files dictionary in the variable (see NOTE below);
• reverts a bunch of changes I made previously;

NOTE:

• if a user wants to use fx files in the diagnostic, all they have to do is include the fx variables in the variables list; they will get the fx files at the end of preprocessing, with any preprocessor of their choice applied to them (and CMOR checked and fixed);
• if a user wants to use mask_landsea or mask_landseaice, all they have to do is to set these preproc steps in the preprocessor; no more need to specify the fx_files in variabl; the code will automatically grab the fx files for masking BUT it will apply the masks from those files WITHOUT doing any CMOR checks/fixes or any preprocessor on the fx files (basically what we had before). The next step will be to implement this functionality (allow preprocessing on fx files that are used in the preprocessor) but that is absolutely non-trival.