There are now three possible code entries for the scalarCodedSource,
vectorCodedSource, etc..., fvOptions. These are `codeAddSup` for basic
sources, `codeAddRhoSup` for compressible sources, and
`codeAddAlphaRhoSup` for phase (compressible) sources.
Previously `codeAddSup` was used for both basic and compressible
sources, and phase sources were not implemented. This meant that whilst
a compressible source could be created, it could not make use of the
`rho` argument or the basic source function would fail to compile.
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