WIP: Fix single precision configuration

[pgrete]

PR Summary

In theory we already supported single precision.
In practice, it didn't work (not even compilation).

Todos:

• Add test coverage for single prec
• Check that all time/timing related parameters are Real64
• Check that all reductions, reduce to Real64
  • kernels
  • MPI
• check use of Real override in unit tests
• Discuss
  • (fixed) type for particles positions
  • (fixed) type for coordinates

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• CI has been triggered on Darwin for performance regression tests.
• (@lanl.gov employees) Update copyright on changed files

[pgrete]

@brryan I encountered some interesting behavior for particles when I tried to compile in single precision.
Most of the position ("x", "y", "z") related function were hard-coded to double, which I understand.
However, the Views themselves were Real resulting in a mismatch when trying to get the data because the map would return Real where doubles are expected in further processing.
Hardcoding the positions to double/Real64 could be an option, but it would require to add just another map next to int and Real.
What's your take on this?

[Yurlungur]

@pgrete @brryan what's the status here?

[pgrete]

Probably still not working and needs fixing.