FITS frontend crashes when parsing units

[ngoldbaum]

Bug report

Bug summary

This was reported by Tazkera Haque on the mailing list.

Code for reproduction

import yt
# http://use.yt/upload/50e098ee
ds = yt.load('signal_wfilt_sn.fits')
ds.index

Actual outcome

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
~/Documents/yt-git-fixes/yt/mods.py in <module>()
----> 1 ds.index

~/Documents/yt-git-fixes/yt/data_objects/static_output.py in index(self)
    502                 raise RuntimeError("You should not instantiate Dataset.")
    503             self._instantiated_index = self._index_class(
--> 504                 self, dataset_type=self.dataset_type)
    505             # Now we do things that we need an instantiated index for
    506             # ...first off, we create our field_info now.

~/Documents/yt-git-fixes/yt/frontends/fits/data_structures.py in __init__(self, ds, dataset_type)
     91         self._handle = ds._handle
     92         self.float_type = np.float64
---> 93         GridIndex.__init__(self,ds,dataset_type)
     94
     95     def _initialize_data_storage(self):

~/Documents/yt-git-fixes/yt/geometry/geometry_handler.py in __init__(self, ds, dataset_type)
     56         # potentially quite expensive, and should be done with the indexing.
     57         mylog.debug("Detecting fields.")
---> 58         self._detect_output_fields()
     59
     60     def _initialize_state_variables(self):

~/Documents/yt-git-fixes/yt/frontends/fits/data_structures.py in _detect_output_fields(self)
    172                     continue
    173                 if self._ensure_same_dims(hdu):
--> 174                     units = self._determine_image_units(hdu.header, known_units)
    175                     try:
    176                         # Grab field name from btype

~/Documents/yt-git-fixes/yt/frontends/fits/data_structures.py in _determine_image_units(self, header, known_units)
    117                     n += 1
    118             if n != len(units): field_units = "dimensionless"
--> 119             if field_units[0] == "/":
    120                 field_units = "1%s" % field_units
    121             return field_units

IndexError: string index out of range

Expected outcome

No crash

Version Information

• Operating System: Mac
• Python Version: 3.5
• yt version: latest master branch
• Other Libraries (if applicable):

[ngoldbaum]

Fixed by #1553