Add PR template to repo

[clizbe]

We have a template for PRs that have some checkboxes. This repo should follow the same checks.
You can find it on the TulipaEnergyModel.jl repo.

[marnoldus]

We have a template for PRs that have some checkboxes. This repo should follow the same checks. You can find it on the TulipaEnergyModel.jl repo.

The PR template references the README.dev.md. Do you suggest copying (relevant parts from) that as well or link to it? And what about the issue templates, do you think these are relevant for this repository as well?