fixing dual laser stimulation

[leoscholl]

• move stimulation_site metadata from othertasks.py to laser_features.py
• set the power for each laser only once at the beginning of the experiment
• add stimulation_site metadata from multi-laser experiments
• add parameters for
  • minimum isi, the minimum space between laser pulses
  • maximum stim time, the time after which stimulation stops
  • random pulse start time, so pulses don't start immediately after the state starts
  • random laser order, so the order of laser stimulation is swapped at random