Error when starting video recording from cameras already recording video

Current behavior throws confusing error when attempting to start a recording on cameras already recording data.

Change E3VisionInterface.start_rec behavior to the following:

check for recording state of all cameras before sending post request
if any cameras are recording, check FORCE parameter
- If FORCE, stop recordings on all cameras then start new recording. Print according warning message
- if not FORCE, raise more meaningful exception with "You're currently recording something" error message.

Please submit this to the e3vision branch.

aolabNeuro / brain-python-interface