REF: expose simpson and shannon

[martinfleis]

Expose the formulas for Simpson diversity and Shannon diversity so these can be used outside their classes to calculate the index for the arbitrary array.

[codecov]

Codecov Report

Merging #183 into master will decrease coverage by 0.11%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   94.21%   94.09%   -0.12%     
==========================================
  Files          11       11              
  Lines        2663     2677      +14     
==========================================
+ Hits         2509     2519      +10     
- Misses        154      158       +4     

Impacted Files	Coverage Δ
momepy/shape.py	96.47% <85.71%> (+0.01%)	⬆️
momepy/diversity.py	93.60% <87.50%> (-1.36%)	⬇️
momepy/utils.py	83.25% <100.00%> (ø)

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