Add centroid, boundary and convex_hull function (returning new Geography)

[jorisvandenbossche]

[codecov[bot]]

Codecov Report

Patch coverage: 44.73% and project coverage change: -1.46 :warning:

Comparison is base (e874e60) 56.07% compared to head (91ce98f) 54.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   56.07%   54.61%   -1.46%     
==========================================
  Files           5        6       +1     
  Lines         214      249      +35     
  Branches       95      114      +19     
==========================================
+ Hits          120      136      +16     
  Misses          7        7              
- Partials       87      106      +19     

Impacted Files	Coverage Δ
src/geography.cpp	41.66% <ø> (-0.27%)	:arrow_down:
src/accessors-geog.cpp	37.93% <37.93%> (ø)
src/geography.hpp	83.33% <50.00%> (-4.17%)	:arrow_down:
src/pybind11.hpp	82.14% <75.00%> (-0.47%)	:arrow_down:
src/spherely.cpp	57.14% <100.00%> (+7.14%)	:arrow_up:

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[jorisvandenbossche]

@benbovy this should be ready I think. There are certainly some things to improve (easier testing, correct return class), but those can be left for follow-ups I think.