Subset type for Reaction Identifier

[bdeadman]

Is your feature request related to a problem? Please describe.
There are times where a dataset includes subsets of reactions which the author may want to designate, for example two tables grouped into 1 dataset, different stages of a DOE campaign.

Describe the solution you'd like
Add a Reaction Identifier type for SUBSET.

Describe alternatives you've considered
The alternative is to have a separate dataset for each subset, but this loses the context that ties them together. This is the approach that has been used in some existing datasets (e.g. ord_dataset-2038a3c967db4a32a8fbce288437e929, ord_dataset-2be30f5d8dcd471aa6ad410bdee05902, ord_dataset-31989f1b2b9d4885b1dd2d9982da4517, ord-0defb78dc9b047179947fa8bf0330132 are 4 datasets from http://doi.org/10.1039/C6RE00153J).

Additional context
I'll setup the PR for ord-schema, but marking it here for discussion.

[bdeadman]

An INDEX type could also be useful, although possibly redundant with the natural order of the reactions in the dataset. Having an INDEX reaction identifier would allow authors to explicitly define the order of reactions in a dataset when this has a specific meaning (defined in the associated details). In contrast, the order of reactions in a dataset does not mean that the data was definitely collected in that order.