DAQ sync square echo in SignalsExp

[k1o0]

Is your feature request related to a problem? Please describe.
Some sync square flips don't work (#199) so reinstating the DAQ Sync Echo could potentially make analysis of these sessions easier.

Describe the solution you'd like
Re-instate the stimWindow object in SignalsExp then use the sync echo functionality of that class. We would have to profile it first to ensure that it's fast enough. The SignalsExpAsSubclass branch goes some way towards this: https://github.com/cortex-lab/Rigbox/tree/SignalsExpAsSubclass

Describe alternatives you've considered
Another option would be to just paste the code into SignalsExp but that's messy. Hopefully we'll fix this sync square issue too but that might be a while.