preliminary date comparison methods

[rlskoeser]

• improved equality check, including support for comparing Undate object with day precision to datetime.date
• implementations for __lt__ , __le__, and __contains__
• tests for comparison, sorting and in (contains)
• static method to initialize an Undate object from a datetime.date, to simplify comparison logic

I'm sure there are cases I'm not handling or assumptions I'm making that don't always hold, so please point out any you can think of and then we can decide whether to fix as part of this PR or create separate issues for additional cases.

[codecov[bot]]

Codecov Report

Merging #65 (fcdd4c2) into develop (ab378da) will increase coverage by 0.27%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #65      +/-   ##
===========================================
+ Coverage    98.65%   98.92%   +0.27%     
===========================================
  Files            4        4              
  Lines          223      280      +57     
===========================================
+ Hits           220      277      +57     
  Misses           3        3              

Files	Coverage Δ
tests/test_dateformat/test_base.py	100.00% <100.00%> (ø)
tests/test_undate.py	100.00% <100.00%> (ø)