Ignore no-data values when scaling band data and allow band selection by color name for Sentinel-2

[lukasValentin]

This PR addresses and closes #86. No-data values are considered, i.e. ignored, when scaling.

In addition, a small bug was fixed allowing now to select Sentinel-2 bands also by color names, as it is already supported for Landsat data:

color_names = ['blue', 'green', 'red']  # is the same as 
band_names =  ['B02', 'B03', 'B04']  # when calling Sentinel2.from_safe

[codecov]

Codecov Report

Merging #87 (0e68be4) into master (e1941b7) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   57.61%   57.58%   -0.04%     
==========================================
  Files          68       68              
  Lines        5290     5300      +10     
  Branches     1060     1063       +3     
==========================================
+ Hits         3048     3052       +4     
- Misses       1940     1943       +3     
- Partials      302      305       +3     

Flag	Coverage Δ
unittests	57.58% <100.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
eodal/core/band.py	75.53% <100.00%> (-0.45%)	⬇️
eodal/core/raster.py	73.16% <ø> (ø)
eodal/core/sensors/landsat.py	70.32% <100.00%> (+0.12%)	⬆️
eodal/core/sensors/sentinel2.py	62.15% <100.00%> (+0.42%)	⬆️

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