Add templates for PR and issue description

[Sheshuk]

We need to add templates with checklists/sections for PR and issues.
This will make it easier to describe the changes

[JostMigenda]

In the template for PRs that add a new SN model, we should make sure to have a checkbox that we’ve verified the normalisation of mu/tau neutrinos. Papers have different conventions there (whether to display each separately or the sum of both) and that has caused bugs a few times (e.g. in #46 and #363).