Respect LISO's plot ordering when plotting a LISO input or output file.
Since solution merging was introduced in 623a9034, colour wheels on plots are no longer reset when plotting LISO equivalents, so colours for equivalent functions are different. Since now the plotting tool doesn't know the difference between functions merged from different solutions, add a new plot spec to functions store a "colour wheel group".
UPDATE 2019/05/07: colours now match when comparing LISO to Zero, but absolute order when running a Zero on its own with a LISO input file still differs from what LISO produces.