New attributes for channel_group metadata

[vhlai-seis]

• Short description of new feature: Adding cable_characteristics, cable_environment and cable_installation_enviroment to channel group metadata
• Submitter: Chris Atherton, Senior Research Engagement Manager, GEANT,

Feature request (reason, suggestion to implement): submarine cables, especially long haul cables, cross many different bathymetric environments. The cable is often build with different armouring in different segments due to the perceived risks, the cables also run along the shore or inland before reaching a cable landing station, so not strictly always in the ocean. Currently the metadata for the cable description is a general description but does not allow for differentiation of the different types of cable armouring or environmental segments. The channel group offers a good opportunity to be able to adequately segment up the cable and provide a better, more accurate description for the cable_characteristics, cable_environment and cable_installation_enviroment.

[chad-earthscope]

This issue is continued at https://github.com/FDSN/DAS-metadata/issues/1