Multi mirror

Implementation of a MultiMirror() ground model that follows the idea of having another ground plane in the sky. Should allow using the concept with any type of deficit model. Also implemented the Ct modification in the RHB model.

Still need to add documentation and test cases, and there is something with the above_ground mask that I could not figure out yet.

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Please use the issue tracker on https://gitlab.windenergy.dtu.dk/TOPFARM/PyWake/-/issues
+title: 'Please use the issue tracker on https://gitlab.windenergy.dtu.dk/TOPFARM/PyWake/-/issues'
+labels: ''
+assignees: ''
+
+---
+
+Please use the issue tracker on https://gitlab.windenergy.dtu.dk/TOPFARM/PyWake/-/issues