ddl.dic: introduce attributes to specify the dictionary licence

[vaitkus]

In the discussion of issue COMCIFS/cif_core#247, it was suggested that it would be useful to have a dictionary attribute for specifying the dictionary licence (e.g. _dictionary.licence).

Alternatively, there could be a separate category for the dictionary licences since authors might be want to specify more than one licence or even link directly to the license text (e.g. _dictionary_licence.url).

Also, it might make sense to put these attributes into the Mandatory category so that the licence would be specified from the start.

[nautolycus]

In the spirit of Keeping It Simple, since SPDX (a) allows multiple licences to be specified in the identifier string, including AND/OR qualifiers, and (b) provides a curated list of recognized licences with text and sources, how about a single _dictionary.licence with definition:

_description.text
; The licence or licences under which this dictionary is distributed. They should be specified using an
  SPDX-License-Identifier: expression (https://spdx.dev/learn/handling-license-info/). If the licence is
  not listed in the curated SPDX list (https://spdx.org/licenses/) then this field may contain the full text
  of the licence. However, registration with the SPDX project and use of an SPDX-License-Identifier: string
  is strongly encouraged.

  Ref: International Organization for Standardization (2021). ISO/IEC 5962:2021. Information technology — 
  SPDX® Specification V2.2.1. Geneva: International Organization for Standardization.
;

loop_
       _description_example.case
         'SPDX-License-Identifier: CC-BY-4.0'
         'SPDX-License-Identifier: CC-BY-4.0 OR CC0-1.0+'