Add support for LeapSecondsFile and UT1

[ChristopherRabotin]

@cjordan This PR adds (initial) support for UT1. It does not make UT1 a TimeScale, but it allows conversion to and from UT1 into other time scales. It also automatically downloads the Earth Orientation Parameters from JPL if needed, or it can read an EOP data from the provided path. This requires the ut1 crate feature.

Closes #43

[codecov-commenter]

Codecov Report

Base: 77.67% // Head: 79.40% // Increases project coverage by +1.72% 🎉

Coverage data is based on head (ae1d94c) compared to base (b1071b9).
Patch coverage: 70.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   77.67%   79.40%   +1.72%     
==========================================
  Files          14       16       +2     
  Lines        3602     3686      +84     
==========================================
+ Hits         2798     2927     +129     
+ Misses        804      759      -45     

Impacted Files	Coverage Δ
src/errors.rs	37.50% <ø> (ø)
src/lib.rs	100.00% <ø> (ø)
src/leap_seconds.rs	60.71% <60.71%> (ø)
src/leap_seconds_file.rs	65.21% <65.21%> (ø)
src/efmt/format.rs	88.88% <100.00%> (+9.33%)	⬆️
src/epoch.rs	87.73% <100.00%> (+0.06%)	⬆️
src/efmt/formatter.rs	74.71% <0.00%> (+24.71%)	⬆️

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[cjordan]

Thanks @ChristopherRabotin ! I'll have a look at massaging this new functionality into our crates when I can.