FIX: reference scene heuristic and uniform pixel size in SceneCollection returned from mapper

[lukasValentin]

This PR addresses and resolves #90:

• we now ensure that all scenes in a SceneCollection returned from a mapper call not only have the same spatial extent but also the same pixel size. This was not necessarily the case before as there was no proper, fixed reference scene used. The problem became evident when reading Sentinel-2 data from different UTM zones. Please note that this is just a technical alignment of grids, we do not check for geolocalisation inaccuracies as we assume the reported geo-referencation to be correct (e.g., from ESA).
• also, we now ensure that spatial reprojection is only called when really necessary

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6ff0a54) 57.79% compared to head (25ff686) 57.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #91      +/-   ##
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+ Coverage   57.79%   57.85%   +0.06%     
==========================================
  Files          68       68              
  Lines        5324     5332       +8     
  Branches     1068     1071       +3     
==========================================
+ Hits         3077     3085       +8     
  Misses       1937     1937              
  Partials      310      310              

Flag	Coverage Δ
unittests	57.85% <100.00%> (+0.06%)	⬆️

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