[cocom] Code complexity by shorter intervals

[svdo]

I'm test-driving the code complexity integration (chaoss/grimoirelab#230). I noticed that the interval for which code complexity is calculated is configured in terms of a number of months. For us it would be much more valuable of we could specify this in terms of a number of weeks, typically '1 week' for us: we track coverage delta on a weekly basis. Thanks for your consideration!