Command line argument to specify index_definition file

In operational use one will want to have different index_definition files, for example to calculate the same index over monthly, seasonal and annual periods. Now climix searches the XDG catalogues and picks the "last definition". It is less convenient, and indeed error-prone, having to change names or location of the definition file just to switch between calculation period, or otherwise index definitions. Thus adding a command line argument specifying the index_definition file (including full path) would be useful.

E.g. something like

climix -f ./path/to/my_project_index_definition.yaml

that overrides the search in the XDG directory hierarchy.