`describe` fails on datasets with `char`-type variables

[nikmolnar]

Trying to use describe (CLI or API) on a dataset with one or more char-type variables causes the following error:

  File "/Users/nikmolnar/projects/trefoil/trefoil/netcdf/describe.py", line 76, in describe
    'min': data.min().item(),
  File "/Users/nikmolnar/virtualenv/trefoil-_zYcxnHA/lib/python3.6/site-packages/numpy/core/_methods.py", line 29, in _amin
    return umr_minimum(a, axis, None, out, keepdims)
TypeError: cannot perform reduce with flexible type

I'm not advocating that we support rendering, or performing other analysis on char variables, but describe should ignore such variables and return information for other variables, instead of failing outright.