[coordinates/source] do not probe h5 file for mdtraj.HDF5TrajectoryFile

[marscher]

The exception triggering and catching was too broad/unspecific to be triggered in certain cases.

Now we just check, if the user wants to read MD data by checking if either a topology or a featurizer has been given.

[codecov[bot]]

Codecov Report

Merging #1378 into devel will decrease coverage by 0.01%. The diff coverage is 94.33%.

@@            Coverage Diff             @@
##            devel    #1378      +/-   ##
==========================================
- Coverage   91.73%   91.71%   -0.02%     
==========================================
  Files         223      223              
  Lines       25244    25271      +27     
==========================================
+ Hits        23157    23178      +21     
- Misses       2087     2093       +6

Impacted Files	Coverage Δ
pyemma/coordinates/data/feature_reader.py	95.21% <100%> (ø)	:arrow_up:
pyemma/coordinates/tests/test_readers.py	99.05% <100%> (+0.03%)	:arrow_up:
pyemma/coordinates/data/util/reader_utils.py	89.18% <90%> (-1.08%)	:arrow_down:
pyemma/coordinates/tests/test_source.py	98.46% <95.83%> (-0.62%)	:arrow_down:
...emma/_ext/variational/solvers/tests/test_direct.py	89.13% <0%> (-8.7%)	:arrow_down:

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[marscher]

Thanks for reviewing!