Make further_info_url attribute only be used if required by CV or requested by user

[mauzey1]

This PR will make CMOR only add a further_info_url attribute to the file if it is either required by the CV, or set in the user input file. If further_info_url is in the required_global_attributes section of the CV, it will require CMOR to add that attribute to the file.

The further_info_url attribute value provided by the user in the input file is a template that will be used by CMOR to generate a URL. If the user doesn't provide a template value when it is required, then CMOR will use its built-in default template for the URL. If the CV file provides a value for further_info_url in the source_id entry, then it will be used as a value for the file's attribute unless further_info_url is set in the user input.

[piotr-florek-mohc]

Looking good to me, and the code behaves as expected, with CMOR complaining about missing attribute if I put further_info_url back into CVs.