Refactored constitutive/fluid folder

[francoishamon]

This PR changes the organization of the constitutive/fluid folder as follows:

fluid
    1. singlefluid (i.e., meant to be used by SinglePhaseBase)
            singleFluidBase
            compressibleSinglePhaseFluid
            thermalCompressibleSinglePhaseFluid
            particleFluidBase
            particleFluid
            slurryFluidBase
            slurryFluid
            + selectors
    2. multifluid (i.e., meant to be used by CompositionalMultiphaseBase)
            multiFluidBase
            PVTDriver
            + selectors
            2.A black-oil
                     blackOilFluidBase
                     blackOilFluid
                     deadOilFluid
                     PVTOData
            2.B CO2-brine
                     CO2BrineFluid
                     2.B.a CO2-brine functions
            2.C compositional
                     PVTPackage wrapper
                     + new compositional files coming
                     2.C.a compositional functions (Rachford-Rice, EOS calculations, etc)
            2.D reactive
                     reactiveMultiFluid
                     2.D.a chemicalReactions

Fixes #2423