Support multiple items per citation-number (for chemistry journals)

We need to be able to support compound reference styles with output like:

(21) (a) Childs, A. F.; Goldsworthy, L. J.; Harding, G. F.; King, F. E.; 
Nineham, A. W.; Norris, W. L.; Plant, S. G. P.; Selton, B.; Tompsett, 
A. L. L. J. Chem. Soc. 1948, 2174–2177. (b) Chase, B. H.; Downes, 
A. M. J. Chem. Soc. 1953, 3874–3877.

My understanding is this conceptually like a footnote citation, where the individual references simply get labeled based on their position in the list.

If I understand that right, then all we need to be able to get the index number for the position within the citation, and then format that using cs:number. Does that mean we need to add something like reference-in-citation-index to the cs-numbers pattern?

Aside: we have first-reference-note-number as a simple variable (not on number). Two things:

1. well, should it have been under number?
2. we probably need to establish consistent naming of three things:
   • the full citation as a whole
   • the individual references within the full citation
   • the item in the bibliography

[bwiernik]

This is definitely not 1.0.2. 1.1 at the earliest, though @fbennett has said he lacks time to build support for it.

[bwiernik]

Here is the most recent discussion of this: https://discourse.citationstyles.org/t/more-complex-citation-bibliography-formats/1588/6