Added sample heater display mode check in custom_pid function.

[mjrand]

Description

I added in a simple check for the display mode of the sample heater in the custom PID function.
If the sample heater is not in "current" mode, then the display mode will be changed to "current" mode.

Motivation and Context

The custom PID function assumes that the sample heater display mode is set to current mode but does not check.
This leads to situations where the sample heater is in power mode and the custom PID will attempt to set the sample heater to a very high number in Watts instead of % of max current. This usually causes the PID function to break because the power in W is over the maximum range of the heater output. This confuses people when the custom PID will seemingly not work when started.

Resolves #494.

How Has This Been Tested?

Simple function addition shouldn't cause issues.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[BrianJKoopman]

This looks good to me. I linked it to the relevant issue, #494. Also just merged in main, since the branch is 8 months old and docs weren't building properly because of it.