reference cell initialised to 0 instead of -1

If you have a cht case with 0 cells on some processors the call to getRefCellValue will fail. The reference cell setting is only done for fields that need it but the getRefCellValue is done always.

In createFluidFields.H replace

List<label> pRefCellFluid(fluidRegions.size(), 0);

with

List<label> pRefCellFluid(fluidRegions.size(), -1);

@Sergio review?