Triple intersection

[dslosky-usgs]

closes #21
closes #18

Fix doc errors and add support for the triple intersection case (demand intersects capacity in three places)

[codecov-io]

Codecov Report

Merging #22 into master will decrease coverage by 0.13%.
The diff coverage is 92.7%.

@@            Coverage Diff            @@
##           master     #22      +/-   ##
=========================================
- Coverage   93.34%   93.2%   -0.14%     
=========================================
  Files          17      17              
  Lines         601     692      +91     
=========================================
+ Hits          561     645      +84     
- Misses         40      47       +7

Impacted Files	Coverage Δ
shakecastaebm/core.py	100% <ø> (ø)	⬆️
shakecastaebm/capacity.py	82.89% <ø> (ø)	⬆️
shakecastaebm/tests/performance_point.py	97.87% <100%> (+2.03%)	⬆️
shakecastaebm/spectrum.py	88.7% <88.88%> (+0.03%)	⬆️
shakecastaebm/performance_point.py	93% <90%> (-4.57%)	⬇️

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