Validation of conflicting measurment lists

[HanBnrd]

• Add validation for the new conditional symbol introduced in Introduce ^ⁿ symbol to snirf_specification.md fNIRS/snirf#166
• Fixes some REQUIRED_DATASET_MISSING validation issues

TODOs:

• Add a test snirf file with both measurementList and measurementLists
• Write a test for the new CONFLICTING_FIELDS_PRESENT error

Ping @sreekanthkura7

@sstucker, we were wondering if introducing a new error code (after the OK code) will be okay with the rest of the validation?

[sreekanthkura7]

@HanBnrd, There were a few issues with the code we wrote earlier, which I’ve now fixed. I created three SNIRF files—one with just measurementList, one with just measurementLists, and one with both—to test the updated code. We can go through the changes and approve the PR in tomorrow’s meeting.