Refactor date format classes

[tilen1976]

Make date format classes handle its own methods and data:

• Add abstract super class DateFormat with abstract methods implemented in subclasses IsoDateFormat and
  SsbDateformat
• Method get_floor returns first date in period (date from)
• Method get_ceil returns last date in period (date until)
• Handle KeyError in SsbDateFormat methods - returns None
• Add tests and doctests
• Remove unnecessary doctests

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
97.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[mmwinther]

Nice work, much easier to follow the code now 🤩