FIX: TimeOfMinute subseconds

[Miksus]

There was a bug in TimeOfMinute subseconds. Also the minutely condition did not work correctly for subseconds as well.

For example these did not work:

• TimeOfMinute("15.2", "45.2")
• TimeOfMinute("15.002", "45.002")

Additional changes:

• Fix: other time periods end (only affects to the last microsecond)
• Add: simpler starting mechanism
• Add: better errors

[codecov-commenter]

Codecov Report

Merging #91 (1ad057e) into master (2413a22) will increase coverage by 0.13%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   91.44%   91.57%   +0.13%     
==========================================
  Files          89       89              
  Lines        4488     4500      +12     
==========================================
+ Hits         4104     4121      +17     
+ Misses        384      379       -5     

Impacted Files	Coverage Δ
rocketry/time/interval.py	98.45% <92.85%> (+2.15%)	⬆️
rocketry/core/time/anchor.py	88.76% <100.00%> (+1.04%)	⬆️

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