geometry

When trying to define an operator in the cling cli repl, the "function definition is not allowed here" error is generated. "cling -version" reports "0.8". Here's a copy/paste of a session to demonstrate the issue:

****************** CLING ******************
* Type C++ code and press enter to run it *
*             Type .q to exit             *
*****************************************

The contributing file is too long and discourages new users. We should:

• split it into several sections, e.g. "what can I do to contribute?" and "Checklist before submitting a PR"
• Add a checklist to the GitHub action that welcomes new contributors to Geomstats.