Check that `data_file` has file extension `.yaml` or `.json`

[GraemeWatt]

Other data file extensions than .yaml and .json are currently accepted by the validator (e.g. .yalm) but these data files are then not included in the tarfile prepared by the hepdata-converter-ws-client package (see this line). Consequently, file conversion fails with a FileNotFoundError error message from this line of the hepdata-converter package. An example is https://www.hepdata.net/record/85727?version=2 (data files with extension .yalm) fixed in https://www.hepdata.net/record/85727?version=3 (data files with extension .yaml).

A "pattern": "^.+\\.(yaml|json)$" should be specified for data_file in the JSON schema:

hepdata-validator/hepdata_validator/schemas/1.1.1/submission_schema.json

Line 26 in bfcd4fd

"data_file": { "type": "string" },