Implement incidence

[blegat]

Closes #67
cc @goretkin

[coveralls]

Coverage increased (+0.04%) to 92.236% when pulling bce4799 on bl/incidence into 9aa7196 on master.

[codecov]

Codecov Report

Merging #73 into master will increase coverage by 0.04%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   92.19%   92.23%   +0.04%     
==========================================
  Files          30       31       +1     
  Lines        1588     1610      +22     
==========================================
+ Hits         1464     1485      +21     
- Misses        124      125       +1

Impacted Files	Coverage Δ
src/redundancy.jl	100% <ø> (ø)	⬆️
src/Polyhedra.jl	100% <ø> (ø)	⬆️
src/elements.jl	84.17% <ø> (-0.54%)	⬇️
src/incidence.jl	100% <100%> (ø)
src/iterators.jl	100% <100%> (ø)	⬆️
src/decompose.jl	79.83% <68.75%> (+0.33%)	⬆️

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