Read dependencies from one file for setup.py and travis CI

[cchwala]

Dependencies are in different files and are inconsistent. As suggested in #71 we read from one requirements file for travis CI and for setup.py.

Not sure what we can do for the environment.yml files.

Closes #71

[codecov-commenter]

Codecov Report

Merging #81 (6083f4b) into master (4957a60) will increase coverage by 2.92%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   62.71%   65.64%   +2.92%     
==========================================
  Files          25       22       -3     
  Lines         818      780      -38     
==========================================
- Hits          513      512       -1     
+ Misses        305      268      -37     

Impacted Files	Coverage Δ
pycomlink/__init__.py	100.00% <ø> (ø)
pycomlink/validation/stats.py	90.12% <0.00%> (+0.12%)	⬆️
pycomlink/validation/validator.py	57.98% <0.00%> (+0.25%)	⬆️
pycomlink/processing/xarray_wrapper.py	92.00% <0.00%> (+0.33%)	⬆️
pycomlink/processing/wet_antenna.py	86.79% <0.00%> (+0.51%)	⬆️
pycomlink/spatial/interpolator.py	75.60% <0.00%> (+0.60%)	⬆️
pycomlink/spatial/idw.py	66.07% <0.00%> (+0.61%)	⬆️
pycomlink/processing/baseline.py	19.40% <0.00%> (+1.22%)	⬆️
pycomlink/util/maintenance.py	73.33% <0.00%> (+1.90%)	⬆️

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