Implement "csv" string expansion

[epruesse]

Array values, e.g. multiple input files, should be expandable as CSV as is often needed for tools accepting many input files.

rule bla:
  input:
    r1 = "{dir}/{:sources:}.{:pairnames[0]:}.fq.gz",
    r2 = "{dir}/{:sources:}.{:pairnames[1]:}.fq.gz"
 params:
    r1  = "{input.r1:,}"
    r2 = "{input.r2:,}"

This probably won't be possible for expansion in shell: directly in a robust manner. It should be doable from within SnakemakeExpander though to have in params etc. Since it would depend on already concretized input, it'd have to run as a function, so essentially limited to params.