Add time_slots and TimeWindow for issue #481

[anurag-mds]

Implements the API proposed in #481 .

time_slots(start, stop, interval; from, to) — generates a TimeArray of timestamps within a daily time window
time_slots(range; from, to) — same, accepts a StepRange{DateTime} directly
when(ta, time_slots(; from, to)) — filters an existing TimeArray by time-of-day via the new TimeWindow type

Closes #481

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.19%. Comparing base (3291223) to head (73a3c44).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   85.44%   86.19%   +0.75%     
==========================================
  Files          11       13       +2     
  Lines         625      898     +273     
==========================================
+ Hits          534      774     +240     
- Misses         91      124      +33     

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