Issue #737 Add PR template (#758)

Fixes #737 

This PR adds a template for PRs. [Unfortunately the template only works
if it is in the default
branch](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/about-issue-and-pull-request-templates#pull-request-templates),
(``master``) in our case, so I could not test it. I presume it works
though, [as it works for
Ribasim](https://github.com/Deltares/Ribasim/blob/main/.github/pull_request_template.md).

[Relevant
docs](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository)

.github/pull_request_template.md

@@ -0,0 +1,20 @@
+Fixes #
+
+# Description
+<!---
+Thanks for opening a PR!
+
+Please add your description here of changes made and how they are going to
+resolve the linked issue 
+-->
+
+# Checklist
+<!---
+Before requesting review, please go through this checklist:
+-->
+
+- [ ] Links to correct issue
+- [ ] Update changelog, if changes affect users
+- [ ] PR title starts with ``Issue #nr``, e.g. ``Issue #737``
+- [ ] Unit tests were added
+- [ ] **If feature added**: Added/extended example