make false_easting and false_northing optional

[Chilipp]

this second part fixes the visualization of the lambert conformal grid in psyplot/psyplot#38. The current implementation assumed the false_easting and false_northing attributes to be mandatory, although they should be optional.

• [x] Tests added (for all bug fixes or enhancements)
• [x] Tests passed (for all non-documentation changes)
• [x] Fully documented, including CHANGELOG.rst for all changes

[codecov[bot]]

Codecov Report

Merging #41 (af0401e) into develop (aa2de37) will increase coverage by 0.04%. The diff coverage is 93.54%.

:exclamation: Current head af0401e differs from pull request most recent head 486a6c0. Consider uploading reports for the commit 486a6c0 to get more accurate results

@@             Coverage Diff             @@
##           develop      #41      +/-   ##
===========================================
+ Coverage    78.36%   78.41%   +0.04%     
===========================================
  Files            6        6              
  Lines         1336     1348      +12     
===========================================
+ Hits          1047     1057      +10     
- Misses         289      291       +2     

Impacted Files	Coverage Δ
psy_maps/plotters.py	85.39% <93.54%> (-0.03%)	:arrow_down:

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