Add singularity EOS and unit conversion

[brbass]

Summary

• Adds singularity-eos as a spack package.
• Adds Spiner, IdealGas, Vinet, and EOSPAC models.
• Adds a UnitConversion class that is used to convert between code and Singularity units (and should be generally useful!).
• Adds tests for the new EOS models and unit conversion.

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ToDo :

• Annotate RELEASE_NOTES.md with notable changes.
• Create LLNLSpheral PR pointing at this branch. (PR#)
• LLNLSpheral PR has passed all tests.