Add nonzero_intervals function

[lpsinger]

This will be important for transforming the field of regard bitmask into interval constraints.

[codecov]

Codecov Report

Merging #64 (e54788c) into main (8d2e373) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   32.15%   32.40%   +0.24%     
==========================================
  Files          30       31       +1     
  Lines        1362     1367       +5     
==========================================
+ Hits          438      443       +5     
  Misses        924      924              

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

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[mcoughlin]

I guess ligo.segments has something similar, but no need to have that package for this!