Updates to Pfeiffer agent.

[tanaybhandarkar]

It now catches when the monitor channels are turned off and prints a statement to the logs. Also sets pressure value to zero if the channel was off. Prior to this, it threw an error when querying pressure values when channels are off

Description

I added an if statement that first queries the status of the channel. If the status is "on", then the agent goes and queries the pressure reading. If the channel status is off, it returns a pressure value of 0.

Motivation and Context

The agent used to crash when trying to query a pressure gauge channel that was off. This error is detailed in Issue #100

How Has This Been Tested?

I have been testing this on the software development computer at Penn. It has been running on this computer over the weekend, with no crashes, despite turning the pressure gauge channels on and off.

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.
• Unless I am preparing a release, I have opened this PR onto the develop branch.

[BrianJKoopman]

Thanks for the PR Tanay, it'll be great to get that error fixed.

While this currently sets the pressure value for any non-OK state to 0, a valid reading that I expect would solve the block mismatch issue, there are a few questions that I have about the return values from the two commands you're using, and some other minor comments to address.