ls372: Add task or argument for setting output mode 'input'

The Lakeshore372 agent doesn't currently have a way to change the Output Mode's 'input' parameter. This specifies which input or channel to control from. This is typically very rarely changed, as a thermometer is placed on the mixing chamber of a DR (or wherever you want to servo), plugged into a specific channel of a 372 and rarely changed. But for initial setup this is important, especially if you are doing that initial setup remotely.

When I initially wrote the agent I was trying to make a separate task for different settings, so there would be a set_heater_input() task or similar. But I could also see this just being an argument to servo_to_temperature() or set_output_mode().

While we're here we might also make the 'powerup enable', 'polarity', 'filter', and 'delay' parameters configurable.

Start by locating the Lakeshore372 agent and its existing output-mode, servo, and heater-setting tasks. Read how those tasks send settings to the instrument, then determine whether the input setting and the listed powerup enable, polarity, filter, and delay parameters should be exposed through an existing task or separate task; done means the required output-mode settings can be configured during initial setup.