Rename 'excentricity' properties to 'eccentricity'

[djhoese]

According to https://en.wiktionary.org/wiki/excentricity "excentricity" is the "Archaic form of eccentricity. This PR renames the properties in the two locations in pyorbital that use it. Everything should be backwards compatible except:

• __str__ now uses the new name
• to_dict now uses the new name

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

❌ Patch coverage is 86.48649% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.35%. Comparing base (860e0aa) to head (45cb9d1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pyorbital/orbital.py	80.00%	3 Missing ⚠️
pyorbital/tlefile.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
- Coverage   90.46%   90.35%   -0.11%     
==========================================
  Files          19       19              
  Lines        3019     3027       +8     
==========================================
+ Hits         2731     2735       +4     
- Misses        288      292       +4     

Flag	Coverage Δ
unittests	90.35% <86.48%> (-0.11%)	⬇️

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[coveralls]

coverage: 90.315% (-0.1%) from 90.421%
when pulling 45cb9d1 on djhoese:feat-eccentricity
into f1c2797 on pytroll:main.

[mraspaud]

LGTM