Record target positions, surface depth in multipatch module

[campagnola]

I think right now we are only recording pipette positions.

Each value needs to be recorded once when the log file is created, and subsequently whenever the value changes.

• Surface height is stored in the Microscope device and can be monitored with Microscope.sigSurfaceDepthChanged. Use man.listInterfaces('microscope') to list the names of all configured microscopes.
• Target positions are managed by individual PatchPipette devices and can be monitored with PatchPipette.sigTargetChanged.

[JFPerkins]

I'm asserting that we will only have 1 microscope. Objection?