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.
Most fvOptions change the state of the fields and equations they are applied to
but do not change internal state so it makes more sense that the interface is
const, consistent with MeshObjects. For the few fvOptions which do maintain a
changing state the member data is now mutable.
