ImageFileCollection: summary is None

[olebole]

The "Working with a directory of images" test in image_management.rstfails for me, both in an automated test and also when running manually:

Python 3.6.6 (default, Jun 27 2018, 14:44:17) 
[GCC 8.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from ccdproc import ImageFileCollection
>>> keys = ['imagetyp', 'object', 'filter', 'exposure']
>>> ic1 = ImageFileCollection('.', keywords=keys)
WARNING: no FITS files in the collection. [ccdproc.image_collection]
>>> ic1.summary.colnames
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'colnames'
>>> type(ic1.summary)
<class 'NoneType'>

• astropy: version 3.0.2
• numpy: version 1.14.3
• ccdproc: 1.3.0.post1

[MSeifert04]

Yeah that test depends on other tests writing a file in that directory. I always forgot to change that.

Thanks for bringing it up.

[mwcraig]

Add sample_data method to ImageFileCollection to populate a directory with files and use those for doc tests.