feat(datafile): Raise IOError when attempting to read empty files

[mwtoews]

This feature provides a more useful error message when attempting to read an empty file. For example, if a mt3dms simulations crashes (e.g. #466), it leaves an empty file.ucn file and this is the error:

ucnobj = flopy.utils.UcnFile('file.ucn')

IndexError: index 0 is out of bounds for axis 0 with size 0

The new message should now be:

OSError: datafile error: file is empty: file.ucn

which is intended to help the user sort out their issue. This message is used for:

• flopy.utils.UcnFile
• flopy.utils.FormattedHeadFile
• flopy.utils.HeadFile (with/without auto-precision detection)
• flopy.utils.CellBudgetFile

[coveralls]

Coverage increased (+0.1%) to 68.699% when pulling a1ad306 on mwtoews:empty into 9f8e632 on modflowpy:develop.

[coveralls]

Coverage increased (+0.1%) to 68.699% when pulling a1ad306 on mwtoews:empty into 9f8e632 on modflowpy:develop.

[langevin-usgs]

Weird things are breaking and we're timing out on 3.7, but they are not related to this, so I'm going to merge it in.