Read Clix meta version and add information in cube output attribute (fixes #284)

• Added dataclass Distribution used by dataclass IndexDefinition in metadata.py. Distribution information (name and version) is read from specified index definition yaml-file.
• Added distribution block to SMHI_extra.yml:

distribution:
  name: SMHI_extra
  version: 1.0

• Added INDEX_DEFINITION and CF_CONVENTIONS_VERSION to global output attribute Conventions template, example output:

:software = "Climix-0.17.0+6.g115aa38" ;
:Conventions = "CF-1.7 clix-meta-0.5.1" ;

Note: the distribution is currently not required to be present in an index definition file, and if missing, will not be included in Conventions.