Added a support for gzipped fie in planetaryimage.

[bvnayak]

refs #42

[wtolson]

It might be nice to implement an interface for handling compression similar to pandas.read_csv(). It takes the following compression argument:

compression: decompress 'gzip' and 'bz2' formats on the fly. Set to 'infer' (the default) to guess a format based on the file extension.

This will allow additional compression formats like bz2 from the stdlib as well as allowing the user to override and disable the compression.

[godber]

@wtolson your suggestion of catching an IOError was a good idea but an error was not being raised, fromfile() was simply returning a null array. So we opted for the explicit route.

Now we just have to figure out how to deal with the python 2.6 and 2.7 issues.

[wtolson]

Interesting, what behaviour are you seeing? On python2.7/numpy1.9.2 I'm seeing this:

In [1]: import gzip

In [2]: import numpy

In [3]: with gzip.open('test.txt.gz', 'rb') as fp:
    data = numpy.fromfile(fp, int, 10)
   ...:     
---------------------------------------------------------------------------
IOError                                   Traceback (most recent call last)
<ipython-input-3-74cf4401a888> in <module>()
      1 with gzip.open('test.txt.gz', 'rb') as fp:
----> 2     data = numpy.fromfile(fp, int, 10)
      3 

IOError: first argument must be an open file

In [4]: print numpy.version.full_version
1.9.2

[godber]

I think Bhavin is workin in python 3 by default. He's in class at the moment.

[bvnayak]

GZIP: Changed in version 2.7: Support for the with statement was added.

BZ2.open is new in Version 3.3