use mambabuild in CI and fix rmsd computation in clustering

[codecov[bot]]

Codecov Report

Merging #1531 (213d21c) into devel (03602c6) will increase coverage by 0.05%. The diff coverage is 98.67%.

@@            Coverage Diff             @@
##            devel    #1531      +/-   ##
==========================================
+ Coverage   92.02%   92.07%   +0.05%     
==========================================
  Files         229      229              
  Lines       26804    26886      +82     
==========================================
+ Hits        24666    24756      +90     
+ Misses       2138     2130       -8     

Impacted Files	Coverage Δ
pyemma/thermo/util/util.py	73.93% <0.00%> (ø)
pyemma/coordinates/api.py	88.11% <87.50%> (-0.11%)	:arrow_down:
pyemma/_base/serialization/tests/test_cli.py	100.00% <100.00%> (+3.33%)	:arrow_up:
pyemma/coordinates/clustering/interface.py	89.25% <100.00%> (ø)
pyemma/coordinates/clustering/kmeans.py	94.42% <100.00%> (+0.09%)	:arrow_up:
pyemma/coordinates/clustering/regspace.py	94.59% <100.00%> (+6.92%)	:arrow_up:
pyemma/coordinates/clustering/tests/test_kmeans.py	98.41% <100.00%> (+0.40%)	:arrow_up:
pyemma/coordinates/data/util/reader_utils.py	87.82% <100.00%> (-0.47%)	:arrow_down:
pyemma/coordinates/tests/test_save_trajs.py	98.87% <100.00%> (+0.19%)	:arrow_up:
... and 6 more

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