First pass at iterable Emissions objects

[altendky]

Fixes #67.

[codecov]

Codecov Report

Merging #72 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          632       634    +2     
  Branches        32        32           
=========================================
+ Hits           632       634    +2     

Impacted Files	Coverage Δ
qtrio/_tests/test_core.py	100.00% <ø> (ø)
qtrio/examples/_tests/test_crossingpaths.py	100.00% <ø> (ø)
qtrio/_core.py	100.00% <100.00%> (ø)
qtrio/examples/emissions.py	100.00% <100.00%> (ø)

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