Problem uploading new media to subjects with existing media

[lcjohnso]

Problem: When calling Subject.add_location() for a subject that has already been created and saved (i.e., has ID + existing media locations entries), new media is not uploaded correctly.

Current Behavior: When initialized (e.g., using Subject.find()), subjects with existing locations entries (N_locations > 0) are initialized with an empty _media_files list (N_mediafiles = 0). When Subject.save() is executed after adding a new subject image via add_location(), the uninitialized _media_files list doesn't match up element-for-element with the locations array leading to issues in the zip() call below:

panoptes-python-client/panoptes_client/subject.py

Lines 158 to 161 in c0bc5c2

	for location, media_data in zip(
	response['subjects'][0]['locations'],
	self._media_files
	):

Furthermore, locations is not added to modified_attributes. But even if modified_attributes is altered manually, the save() will fail when the mismatch leads to the client trying to PUT to an existing, unwritable location.

Here's an illustrative example:

s = Subject.find(<SUBJECT_ID>)                                              

In [7]: len(s.locations)                                                        
Out[7]: 3
In [8]: len(s._media_files)                                                     
Out[8]: 0

s.add_location('kitten.jpg')                                            

In [10]: len(s.locations)                                                       
Out[10]: 4
In [11]: len(s._media_files)                                                    
Out[11]: 1

Desired Behavior: Initialize Subject() instances with a _media_files array whose length matches that of locations array and is filled with None elements. Also, add locations to modified_attributes when add_location is called. Fixing both of these issues solves upload problem.