Standardize multiple footnotes

[cmc333333]

27 CFR 555 changes from

<SU>3</SU>
              <SU>4</SU>

to

<SU>3,4</SU>

and

<SU>4</SU>
              <SU>5</SU>

becomes

<SU>4,5</SU>

in the 2007 edition. Currently, we split a <SU> but retain all of the separators. Let's just standardize sequences of footnotes into a sequence, e.g. <SU>3</SU> <SU>4</SU> <SU>5</SU>

[cmc333333]

@tadhg-ohiggins iirc, this is required for #37, so I'm assigning to you

[tadhg-ohiggins]

So

<SU>2, 3, 4</SU>
<SU>2,3,4</SU>
<SU>2</SU>, <SU>3</SU>, <SU>4</SU>
<SU>2</SU><SU>3</SU><SU>4</SU>

(and other similar variants) should all turn into:

<SU>2</SU> <SU>3</SU> <SU>4</SU>?

[cmc333333]

I think so. I don't know that we explicitly need to account for your third case, but it couldn't hurt. An alternative would be to manually modify the XML to make them consistent, but given that we're already separating <SU>2,3,4</SU>, it seems like we could just standardize it all as a preprocessing step.