added _rewrite_time_axis and refactoring of cmor module

[larsbuntemeyer]

This PR adds management of CF compliant time axis and bounds. Also did some refactoring to prepare for other cmor backend.

closes #79

[codecov-commenter]

Codecov Report

Merging #92 (2376f99) into master (781cc40) will decrease coverage by 1.17%.
The diff coverage is 69.11%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   78.30%   77.14%   -1.17%     
==========================================
  Files          21       22       +1     
  Lines        1037     1098      +61     
==========================================
+ Hits          812      847      +35     
- Misses        225      251      +26     

Flag	Coverage Δ
unittests	77.14% <69.11%> (-1.17%)	⬇️

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Impacted Files	Coverage Δ
cordex/tutorial.py	63.49% <ø> (ø)
cordex/cmor/cmor.py	64.44% <61.45%> (-4.73%)	⬇️
cordex/cmor/config.py	75.00% <75.00%> (ø)
cordex/cmor/utils.py	91.17% <95.45%> (+1.40%)	⬆️
cordex/cmor/__init__.py	100.00% <100.00%> (ø)