Read single precision fluid field in RAMSES

[AnatoleStorck]

PR Summary

The current back-end for reading RAMSES fluid fields assumes the data is double precision. This PR (in collaboration with @cphyc ) is an initial expansion of the RAMSES fluid reader to read single precision fields. For my simulations, the RT fields are written in single precision so this change is needed to read them in.

PR Checklist

• [x] New features are documented, with docstrings and narrative docs
• [ ] Adds a test for any bugs fixed. Adds tests for new features.

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[cphyc]

For reviewers: testing is complicated as all the RAMSES datasets at hand are stored in double precision!