Improve issue raising process

[thompson-tomo]

Having recently worked through raising a number of issues, I feel we could improve this process and hopefully triage issues more effecently. To do this I feel it would be beneficial to replace the current options when raising issues (see below) with more focused options.

The options for me would be:

• Add additional option for a list of support values
• Introduce new attribute to an existing group
• Request an additional metric
• Suggest a change to an existing definition
• Propose a new convention group

Each of these would have 1 or more labels assigned automatically to the issues