[Iterable] fix default_chunksize calculation.

[marscher]

I think we need to release 2.5.1...

[codecov]

Codecov Report

Merging #1247 into devel will increase coverage by 0.01%.
The diff coverage is 83.87%.

@@            Coverage Diff             @@
##            devel    #1247      +/-   ##
==========================================
+ Coverage   91.19%   91.21%   +0.01%     
==========================================
  Files         219      219              
  Lines       23623    23643      +20     
==========================================
+ Hits        21543    21565      +22     
+ Misses       2080     2078       -2

Impacted Files	Coverage Δ
...mma/coordinates/tests/test_coordinates_iterator.py	99.04% <100%> (+0.03%)	⬆️
pyemma/coordinates/data/_base/transformer.py	91.24% <50%> (-1.36%)	⬇️
pyemma/coordinates/data/_base/iterable.py	96.68% <88.23%> (-0.52%)	⬇️
pyemma/coordinates/data/util/traj_info_backends.py	91.54% <0%> (-1.88%)	⬇️
pyemma/coordinates/transform/vamp.py	90.38% <0%> (+0.07%)	⬆️
pyemma/coordinates/transform/pca.py	93.91% <0%> (+1.73%)	⬆️
pyemma/msm/estimators/maximum_likelihood_hmsm.py	86.04% <0%> (+3.25%)	⬆️

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[marscher]

approved offline by @clonker