Cleanup Performance engine

[huppd]

The performance engine parses ICON log files and puts them in a TimeTree data structre.

It should be improved in the following ways:

• rename timing_regex option. For example log-file.
• The regex is hardcoded in util/icon/extract_timings.py. Whenever a machine (or user) uses a different date format, it will give the error "Could not match any regex for start and end time". Make it more rubust.
• The help description for "timing-database" says that this is pointing to a persistent file. Apart from that it is not just one file, but three files, this also doesn't hold true, if the performance files are not being created as reference but for the performance-check. Could we have a better help description there?