starting to refactor reports into classes, also refactored some more dimensions which is still not 100% working and needs more work

starting to move dimensions from core into plugins and put each dimension into its own class. This is very experimental right now. More experimental stuff will follow

A nice creation by @sgiehl

@iquito thanks for the tip, it is a better solution!

Let's see if some test will fail as a result.

in particular adding instruction to restart web server after having updated piwik.
This should help limit number of email reporting errors, as many of these errors are caused by OPcache being not updated.

 (not sure why this has changed in last couple days, no related code was changed)"

Optimizations for import_logs.py Fixes #5314

Kuddos for nice pull request! We hope to see more contributions from you in the future :)

As suggested by cbay, the cache key can be a tuple instead of a string
concatenation.