BlackoilFluidSystem: use int for component indices

[akva2]

The one/two phase indices class uses negative values for inactive components (though not really relevant, but it's a source of confusion). Likewise, the GenericOilGasFluidSystem uses ints.

To avoid nasty casts in generic code, make the fluidsystems consistently use int, even though the blackoil indices are always positive.

[akva2]

jenkins build this opm-simulators=5950 please

[akva2]

These indices in the BlackoilFluidSystem is a bit confusing for one/two-phase. One would get the impression that the BlackoilFluidSystem would use different traits for this, but this is not the case. It's always using the default index traits, while the Indices typetag class is a translation layer from the canonical indices used in the fluid system. It would be probably be a good idea to rename the members in the fluid system to better reflect this and potentially get rid of the Traits entirely.