Usability enhancements, especially for Python

[ChristopherRabotin]

• Epoch and Duration now have a min and max function which respectively returns a copy of the epoch/duration that is the smallest or the largest between self and other, cf. #164.
• [Python] Duration and Epochs now support the operators >, >=, <, <=, ==, and !=. Epoch now supports init_from_gregorian with a time scape, like in Rust. Epochs can also be subtracted from one another using the timedelta function, cf. #162.
• TimeSeries can now be formatted in different time scales, cf. #163

Closes #162
Closes #163
Closes #164

[codecov-commenter]

Codecov Report

Base: 78.88% // Head: 78.58% // Decreases project coverage by -0.30% ⚠️

Coverage data is based on head (498fb45) compared to base (79fa195).
Patch coverage: 65.82% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
- Coverage   78.88%   78.58%   -0.31%     
==========================================
  Files           9        9              
  Lines        2363     2437      +74     
==========================================
+ Hits         1864     1915      +51     
- Misses        499      522      +23     

Impacted Files	Coverage Δ
src/duration.rs	83.97% <0.00%> (-1.01%)	⬇️
src/timeunits.rs	84.25% <ø> (ø)
src/epoch.rs	87.31% <26.66%> (-0.76%)	⬇️
src/timeseries.rs	89.03% <88.88%> (-0.08%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.