Add option to pass one or multiple STASHmaster files for the variable translation structure

[atteggiani]

Currently, the translation of STASH itemcodes (LBUSER(4) in the UMDP F03 specification) into variables is carried out using the atm_stashvar in the stashvar_cmip6.py file.

#158 adds the functionality to select a model passing the --model option to the CLI and select some options to override the "default" atm_stashvar.
Although this is a good way to allow pre-set translation structures (for example `ACCESS-CM2, ACCESS-ESM1.5, ACCESS-ESM1.6, etc.), it might not be enough for files coming from model/experiments using a custom (or different) set of STASHmaster files (main STASHmaster and optional additions).

This issue would be solved by allowing a STASHmaster file to be passed directly to the option (that I would then rename --stashmaster).
We could still maintain the pre-set options currently in place.

In that way I propose the following:

• rename the --model option to --stashmaster;
• the (renamed) --stashmaster option can accept a pre-defined set of strings (the one used currently) or a series of paths that define the STASHmaster files to read.