Rename relaton caches

[opoudjis]

From global ? "#{Dir.home}/.relaton-bib.pstore" : "#{@filename}.relaton.pstore"

to a suffix of @ronaldtse's choice.

[ronaldtse]

• Global cache: ~/.relaton/cache
• Local cache (relative to current directory): relaton/cache

I don't think we should incorporate the @filename because it prohibits re-use if there are multiple files in the same directory.

[opoudjis]

I'm surprised; I'd have thought you'd want the local cache specific to the document and not the directory. In fact, this change means I did not need to do metanorma/metanorma-cli#9 after all. But OK.

[opoudjis]

Need to change relaton-cli as well, since it uses the current cache.