Add slew_time function

[lpsinger]

This is not yet used anywhere, but we will need it to implement slew time constraints.

[codecov]

Codecov Report

Merging #63 (e791995) into main (67eb066) will increase coverage by 0.10%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   32.05%   32.15%   +0.10%     
==========================================
  Files          29       30       +1     
  Lines        1357     1362       +5     
==========================================
+ Hits          435      438       +3     
- Misses        922      924       +2     

Impacted Files	Coverage Δ
dorado/scheduling/_slew.py	50.00% <50.00%> (ø)
dorado/scheduling/__init__.py	100.00% <100.00%> (ø)

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[mcoughlin]

Yes good to have this in here for dev purposes.