Add two new filter sets for [S II]

First stab at the null hypothesis:
- 6716/6731 from FQ672N, F673N, F547M
- 6731/6716 from FQ674N, F673N, F547M
Then multiply them together, which should give unity.

This didn't look promising.  Try synthetic double filter next.
Add ability to use a synthetic composite filter

This is only half the battle.  We also need to be able to specify a
multiplet line, with (supposed) known ratio.  Look at Nebulio for
inspiration.
Describe Balmer decrement plot in the fig caption