Only pass copy of trajectory list to mdtraj.load and (CI) update macos image

[codecov[bot]]

Codecov Report

Merging #1526 (85650aa) into devel (03602c6) will increase coverage by 0.05%. The diff coverage is 97.14%.

@@            Coverage Diff             @@
##            devel    #1526      +/-   ##
==========================================
+ Coverage   92.02%   92.07%   +0.05%     
==========================================
  Files         229      229              
  Lines       26804    26833      +29     
==========================================
+ Hits        24666    24707      +41     
+ Misses       2138     2126      -12     

Impacted Files	Coverage Δ
pyemma/coordinates/clustering/tests/test_kmeans.py	98.00% <ø> (-0.01%)	:arrow_down:
pyemma/thermo/util/util.py	73.93% <0.00%> (ø)
pyemma/coordinates/data/util/reader_utils.py	87.82% <83.33%> (-0.47%)	:arrow_down:
pyemma/coordinates/api.py	88.11% <87.50%> (-0.11%)	:arrow_down:
pyemma/__init__.py	84.78% <100.00%> (ø)
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/data/feature_reader.py	95.50% <100.00%> (+0.02%)	:arrow_up:
... and 7 more

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[clonker]

the failing job didn't actually fail but just timed out in the last step, merging this