Add headers to axis and formula terms files and add checksums to headers

[mauzey1]

NOTE: The CI tests are failing since the current nightly version of CMOR doesn't support the new headers.

This PR adds headers to CMIP6_coordinate.json and CMIP6_formula_terms.json as well as updating some of the header info in CMIP6_grids.json. It also adds checksums to headers computed from the JSON strings produced from the table JSON without the checksum attribute in the header. I used the following function to create these checksums.

import json
import hashlib

def create_md5(table_path: str) -> None:
    table: dict
    with open(table_path) as f:
        table = json.load(f)

    # Remove existing checksum if present
    prev_checksum: None|str = None
    if 'checksum' in table['Header']:
        prev_checksum = table['Header'].pop('checksum')
        print(f'{table_path} has checksum of {prev_checksum}')

    table_str = json.dumps(table, sort_keys=True)
    checksum = hashlib.md5(table_str.encode("utf8")).hexdigest()
    table['Header']['checksum'] = checksum

    if prev_checksum is not None and prev_checksum == checksum:
        print(f'{table_path} checksum unchanged')
    else:
        print(f'{table_path} has new checksum of {checksum}')
    
    with open(table_path, 'w') as f:
        json.dump(table, f, indent=4, separators=(", ", ": "))
        f.write('\n')

[durack1]

@mauzey1 let's pick this up on Tuesday 2nd Sept