add `scan_rules`: report possible improvements

[edwindj]

Add a scan_rules/detect_rules function that reports on possible improvements on the rule set.

This includes:

• feasiblity

• contradictory rules and their couterparts

• redundant rules and their implicators

• variable ranges (e.g. also for categorical variables).

Open question: what should the result be? Print out on screen? Or a markdown file (or something like that?). Html notebook, may be a good option.