Add ability to pass sample_heater param to LS372Agent's `acq()` process

[BrianJKoopman]

The LS372's acq() process can take the parameter 'sample_heater', which optionally reads out the sample heater output. This is off by default, and is typically toggled via the SCF, but it'd be great if users could set this in ocs-web. That would allow them to enable the output if it's disabled, without modifying the SCF.