--semisupervised mode fails when no tracks are specified

[EricR86]

Original report (BitBucket issue) by Francis Nguyen (Bitbucket: Francis Nguyen).

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When running segway train with --supervised, but without specifying --tracks-from, Segway v2.0.2 crashes with the following error:

Traceback (most recent call last):
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/bin/segway", line 11, in <module>
    sys.exit(main())
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/run.py", line 3743, in main
    return runner()
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/run.py", line 3508, in __call__
    self.run(*args, **kwargs)
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/run.py", line 3458, in run
    self.save_gmtk_input()
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/run.py", line 2053, in save_gmtk_input
    self.save_structure()
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/run.py", line 1958, in save_structure
    self.clobber)
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/_util.py", line 156, in __call__
    return save_template(filename, self.resource_name, self.make_mapping(),
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/segway/structure.py", line 89, in make_mapping
    max_num_datapoints_track = num_datapoints.max()
  File "/mnt/work1/users/home2/fnguyen/miniconda3/envs/py2.7.3/lib/python2.7/site-packages/numpy/core/_methods.py", line 26, in _amax
    return umr_maximum(a, axis, None, out, keepdims)
ValueError: zero-size array to reduction operation maximum which has no identity

Since --tracks-from does not have to be specified when not using --semisupervised, this appears to be a bug.

Current workaround is just to remember to specify --tracks-from when using --semisupervised.