adds code intention comments and TODOs

[rsueppel]

In think I understood most of the code, but am not 100% sure.
This has a number of the TODOs, but only a few major ones.

• The repetive code parts should be combined.
• Split dates and sample start dates should be clearer disentangled.

The minimum logic of interval learning must work as follows:

The first valid training sample must satisfy both min_cids and min_periods conditions.
However, the start date can be before the first date for which min_cids is satisfied,
We do not want to waste the experiences of countries with long history.
The max_periods condition may apply.

Please add a little commentary to explain the intentions of code sections
Also always inform about what methods return with return keyword.