Add index back to DMRS

[goodmami]

In an email, Ann said it makes sense to include INDEX in DMRS representations. This would take the form of a graph attribute. E.g.:

<dmrs index="10002" ...>...</dmrs>

In the future, TOP might be handled the same way, but for now the /H link from nodeid 0 to the top node is ok.

[goodmami]

This requires the following:

• Add to DMRX serialization format
• Add to SimpleDMRS serialization format
• Add to DMRS JSON format (or rather, DMRS dictionary views)
• Add index parameter to DMRS class (also top parameter? what to do if there's a top link, too?)

edit: removed DMRS PENMAN from list because it wouldn't be representable in the s-expression view, except perhaps as a node property.