Enable building and testing Omega in single precision

[mwarusz]

This PR enables configuring Omega to use single precision and adds a test (reusing the tendency terms test) checking that we can build and run in single precision. The main changes are:

• Changing R8 to Real where appropriate
• Reading mesh and state arrays into temporary R8 arrays before storing them into the class members
• Always using R8 buffers for halo exchanges
• CMake logic to build the single precision library when needed

The halo buffers change is not optimal for performance in single precision. It would need to be optimized if we decide
to pursue this option seriously in the future.

Checklist

• Documentation:
  • User's Guide has been updated
  • Developer's Guide has been updated
  • Documentation has been built locally and changes look as expected
• Testing
  • CTest unit tests for new features have been added per the approved design.
  • Unit tests have passed. Please provide a relevant CDash build entry for verification.