Revamp conversions and use instances instead of types as parameters

[helgee]

This needs JuliaLang/METADATA.jl#13963

Once I have documented the @transform macro the package should (finally) be ready for publication 🎉

[codecov-io]

Codecov Report

Merging #16 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   97.28%   97.41%   +0.12%     
==========================================
  Files           6        6              
  Lines         295      271      -24     
==========================================
- Hits          287      264      -23     
+ Misses          8        7       -1

Impacted Files	Coverage Δ
src/AstronomicalTime.jl	100% <ø> (ø)	⬆️
src/TimeScales.jl	100% <100%> (ø)	⬆️
src/conversions.jl	100% <100%> (+0.56%)	⬆️
src/Epochs.jl	97.14% <100%> (-0.23%)	⬇️
src/Periods.jl	86.95% <0%> (-1.51%)	⬇️
src/LeapSeconds.jl	100% <0%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.1%) to 97.417% when pulling 5768ba0 on transform into fe85130 on master.

[helgee]

I want to make one more change and add the argument for the conversion function, e.g. ep, as a parameter for the macro:

@transform TAI UTC ep begin
     jd1, jd2 = julian1(ep), julian2(ep)
     date, date1 = ERFA.taiutc(jd1, jd2)
     UTCEpoch(date, date1)
 end

Makes it a bit less magical and you do not need to memorize what arguments the resulting function will accept.