Pull request template

[mellis13]

This PR creates a PR template for SmartSim. The PR template is meant to give more structure and consistency to the developer submitting the PR and the developers reviewing the PR.

It is worth noting that only a single PR template was created. That is, a separate PR template was not created for bugs, features, documentation updates, etc. This is because when there is more than one template, there is no way to access the various templates via the web GUI -- it can only be accessed through manipulation of the site URL query parameters (https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository). To make things easier for new contributors, it was decided to stick with one PR template which can be loaded automatically without query parameters.

The proposed template will not be visible in this repository until it is merged. However, I have pushed the changes to my fork's develop branch (https://github.com/mellis13/SmartSim), and as a result, PRs against my fork of SmartSim will show the rendered template.