Remove raise exception in eye tracking processing behavior only sessions

[morriscb]

Currently, the BehaviorSession object can process behavior only sessions via setting the skip_eyetracking variable to True. Instead of relying on this to process behavior only sessions (of which there are many) we can enable more graceful processing of these data by automatically checking for the existence of eye tracking data, returning None as in the current case of skip_eyetracking=True.

• [x] Add code to BehaviorSession in from_lims, from_json, and from_nwb to gracefully handle cases with no eye_tracking data.
• [x] Remove the skip_eyetracking variable.
• [x] Test that processing runs as expected on the full set of behavior only data from VBO and VBN to make sure there are no "special cases".

[morriscb]

I looked over Scott's scripts for generating the input json files and nwb files to test behavior only data. Unfortunately, most of it is coded to work on ecephys session ids as input. So, in order to get to a point where we can test behavior only vbn data against the code we'd have to write extra code and scripts to generate the input json data. I propose then testing this on VBO (already did a behavior only json file will test the rest through the lims api) only for now and then updating the code if we run into issues with the any edge case VBN data. Sound good @aamster and @danielsf?

[aamster]

@morriscb sounds good.