Implement full GE2/1 front-end environmental variables monitoring

Description

This MR adds the temperature, low voltages, and current monitoring for GE2/1 to the monitoring suite.

Values taken from the GE2/1 OHv2 specifications.

Related Issue

Closes #189 (closed)

How Has This Been Tested?

Pending tests for GE2/1 (with the demonstrator at p5?) No regressions on the GE1/1 integration setup.

Please review carefully for any typo!

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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• All new and existing tests passed.