Add obsmicro case for weekly data

[atecon]

In principle weekly data can be grouped by various dimensions:

1. year by month (the obsmajor case)
2. month by year (the obsminor case)

as well as

3. year by calendar week (not supported, would be the obsmicro case)
4. calendar week by year (another case)
   etc.

Currently (v0.5) only cases 1 and 2 are supported, meaning that calling type "obsmicro" results in an error for weekly data.