[plots1d] mask zero-counts in logscale feature histogram

[thempel]

contributes to https://github.com/markovmodel/pyemma_tutorials/issues/100

[codecov[bot]]

Codecov Report

Merging #1340 into devel will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##            devel    #1340      +/-   ##
==========================================
- Coverage   91.71%   91.71%   -0.01%     
==========================================
  Files         224      224              
  Lines       25151    25153       +2     
==========================================
+ Hits        23068    23069       +1     
- Misses       2083     2084       +1

Impacted Files	Coverage Δ
pyemma/plots/plots1d.py	88.63% <100%> (+0.54%)	:arrow_up:
pyemma/coordinates/clustering/tests/test_kmeans.py	97.89% <0%> (-1.58%)	:arrow_down:
pyemma/plots/networks.py	90.38% <0%> (+0.96%)	:arrow_up:

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