Update Catch to Catch3

[joto]

We are currently using Catch2 which has not gotten any maintainance in years. Development has moved to Catch3. At some point we need to switch to that.

Issues:

• Catch3 is not header-only any more. So we need to update our build system. Just including the header in our repo isn't going to be enough. We need to use an external lib.
• Any syntax and other changes between Catch2 and Catch3 need to be dealt with.

See https://github.com/catchorg/Catch2 and https://github.com/catchorg/Catch2/blob/devel/docs/migrate-v2-to-v3.md