Better caching and coding for each metric calculation

[Akronix]

Each selection of metric x wiki should be cached individually instead of caching as a whole the {selected wikis x selected metrics} set, so that subsets of that current selection are cached as well.

In order to do this, likely, the call for computing metrics will need to be improved. See this line of code for a starting point: https://github.com/Grasia/WikiChron/blob/master/lib/interface.py#L60