doc: Investigated smart- and auto-remove behavior

[buhtz]

Related to #1945

Documentation of my research about how the smart- and auto-remove of BIT is working.

• The docu is temporary and will result in revising the behavior including an update of the user manual and the test suite.
• Improved doc strings.
• Fix minor bug: smartRemoveKeepAll() now check dates and not snapshot IDs. That caused inconsistent behavior with keep-rule on day level.
• Refactor: increment and decrement months
• New unit tests.

Next steps after this PR is merged:

• I will start refactoring code with the goal to add unit tests related to the auto/smart remove behavior.
• I also have ideas how to improve the behavior, the GUI and user manual. See this mockup. I will announce this also on the mailing list and to the community.