Save VFAT configuration in JSON file at run start

Description

This MR aims at collecting the configuration parameters of the VFATS, as the latency and threshold (DAC), at the end of the configuration step, and providing them in a json file that is produced in the directory runXXXXXX/ at the start of each run.

Related Issue

Closes #183 (closed)

How Has This Been Tested?

The json file produced for the coffin is:

{
    "crate": {
        "1": {
            "slot": {
                "2": {
                    "link": {
                        "1": {
                            "vfat": {
                                "0": {
                                    "LATENCY": 173,
                                    "THRESHOLD_DAC": 44
                                },
                                "1": {
                                    "LATENCY": 173,
                                    "THRESHOLD_DAC": 76
                                },
                                "10": {
                                    "LATENCY": 173,
                                    "THRESHOLD_DAC": 35
                                },
... 

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.