Feature/allow specification of schema directory

[hb-bodc]

This adds a new code path to allow a user-specified schema validation file to be specified.

• It modifies the validate method to add a new optional parameter called schema_path.
• If not provided (so it is set to None by default) the code will validate against the standard Argo validators.
• If supplied, it will use the schema file at schema_path in place of the Argo validation schemas.

So existing code should not be affected at all.