Add GE1/1 VFAT FEAST voltage monitoring

Description

The title should speak for itself.

Also includes various bug fixes or enhancements related to the SCA-based monitoring.

• Ensure that the ADC conversions are over before retrieving the measurement, even in the case of an AXI bus error.
• Make use of the 1kOhm precision resistor to calibrate the ADC current source (improve dramatically the PT100/PT1000 precision).
• Export the raw ADC counts in the JSON monitorable object for debugging purposes.

Related Issue

None... 😕

How Has This Been Tested?

Thoroughly tested on the GE1/1 QC7 setup.

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.