Add default lat/lon precision to base Geometry class

[rayrrr]

In response to #84:

• Add a precision parameter to the Geometry base class (accessible on all GeoJSON Type subclasses)
• Set default precision to 6 decimal places (~11.1cm)
• Update unit tests to confirm rounding feature
• Update examples in docs to explain rounding feature

[codecov]

Codecov Report

Merging #131 into master will decrease coverage by 2.89%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #131     +/-   ##
=========================================
- Coverage   95.36%   92.46%   -2.9%     
=========================================
  Files          10       10             
  Lines         345      345             
  Branches       69       69             
=========================================
- Hits          329      319     -10     
- Misses          9       16      +7     
- Partials        7       10      +3

Impacted Files	Coverage Δ
geojson/geometry.py	97.64% <100%> (-2.36%)	⬇️
geojson/mapping.py	80.95% <0%> (-9.53%)	⬇️
geojson/utils.py	84.76% <0%> (-3.81%)	⬇️
geojson/base.py	94.64% <0%> (-3.58%)	⬇️

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[codecov]

Codecov Report

Merging #131 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files          10       10           
  Lines         345      345           
  Branches       69       69           
=======================================
  Hits          329      329           
  Misses          9        9           
  Partials        7        7

Impacted Files	Coverage Δ
geojson/geometry.py	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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