cryptic error message when chromsizes is not formatted properly

[sergpolly]

@nandankita discovered this recently:

that's the error one would get :

Command error:
  + cooler cload pairix --nproc 9 --assembly gal5 gal5Allele.chrom.sizes:1000 MNP-DT40-1-3-3-R1-T1__gal5.nodups.pairs.gz MNP-DT40-1-3-3-R1-T1__gal5.1000.cool
  /miniconda3/lib/python3.6/importlib/_bootstrap.py:219: RuntimeWarning: numpy.dtype size changed, may indicate binary incompatibility. Expected 96, got 88
    return f(*args, **kwds)
  Traceback (most recent call last):
    File "/miniconda3/bin/cooler", line 11, in <module>
      sys.exit(cli())
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 722, in __call__
      return self.main(*args, **kwargs)
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 697, in main
      rv = self.invoke(ctx)
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
      return _process_result(sub_ctx.command.invoke(sub_ctx))
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
      return _process_result(sub_ctx.command.invoke(sub_ctx))
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 895, in invoke
      return ctx.invoke(self.callback, **ctx.params)
    File "/miniconda3/lib/python3.6/site-packages/click/core.py", line 535, in invoke
      return callback(*args, **kwargs)
    File "/miniconda3/lib/python3.6/site-packages/cooler/cli/cload.py", line 248, in pairix
      chromsizes, bins = _parse_bins(bins)
    File "/miniconda3/lib/python3.6/site-packages/cooler/cli/cload.py", line 47, in _parse_bins
      bins = util.binnify(chromsizes, binsize)
    File "/miniconda3/lib/python3.6/site-packages/cooler/util.py", line 261, in binnify
      ignore_index=True)
    File "/miniconda3/lib/python3.6/site-packages/pandas/core/reshape/concat.py", line 225, in concat
      copy=copy, sort=sort)
    File "/miniconda3/lib/python3.6/site-packages/pandas/core/reshape/concat.py", line 256, in __init__
      objs = list(objs)
    File "/miniconda3/lib/python3.6/site-packages/cooler/util.py", line 249, in _each
      n_bins = int(np.ceil(clen / binsize))
  ValueError: cannot convert float NaN to integer

in case you screw up formatting in your .chrom.sizes, for example like this(pay attention to the hidden characters):

this was too permissive:
https://github.com/mirnylab/cooler/blob/4ca715acfeb4b6f2d118589db5d1900339810f37/cooler/util.py#L197
and interpreted allele1 111111 as a chrom name with the corresponding "size" - NaN ...

• Maybe enabling pd.read_csv(...verbose=True...) would have been helpful ?
• There probably shouldn't by ANY NaNs in the .chrom.sizes file, could we leverage that - to catch such mistakes earlier ?
• Maybe this is all too "exotic" of a case, cause people usually use this tools with standard genomes - not so sure ...

[nvictus]

See #209