fieldMinMax: Added cell index to output

e.g.

fieldMinMax fieldMinMax write:
    min(T) = 291 in cell 255535 at location (-0.262546 -0.538933 1.00574) on processor 9
    max(T) = 336.298 in cell 419031 at location (1.7468 0.758405 8.10989) on processor 1
    min(mag(U)) = 0 in cell 14990 at location (-0.0824383 1.68479 1.5349) on processor 0
    max(mag(U)) = 652.341 in cell 218284 at location (0.609849 0.167247 1.00091) on processor 12