Use .cache/PROPOSAL/ for proposal tables and create directory

[maxnoe]

Proposal uses $HOME/.local/share/PROPOSAL/tables for interpolation tables.
Currently, the user has to create this directory to enable table creation.

I think it would be better to use $HOME/.cache/PROPOSAL as it would fit better for this purpose and to automatically create this, if it does not exist.

[Jean1995]

What would be an appropriate default value if the user does not define a folder to save the interpolation tables?

On our development branch, the tables are built to /tmp.
One could also argue that per default, the tables should be built only inside the memory and not be saved to a file at all (similar to the treatment in old PROPOSAL versions).

[maxnoe]

~/.cache/proposal is the correct location for cached things. And since it is highly likely a user will run proposal more than twice, it is very valid to use a file cache by default.

You have to make sure that cache is correctly invalidated though, e.g. by including version and other needed information in the filenames.

And maybe a function like clear_cache would be good just for convenience.