Search Enrichment: Add new/advanced filters based on div/@subtype

[WaxCylinderRevival]

Current @subtype value	frequency
about-frus-series	27
acknowledgements	2
additional-volumes	37
appendix	28
chapter-introduction	8
editorial-note	8,289
editorial-policies	4
errata	38
errata_document-numbering-error	2
event	38
graphic-materials	10
historian-statement	1
historical-document	276,046
index	1,130
maps	1
notes	2
note-on-covert-actions	34
preface	378
press-release	43
referral	574+
related-materials	2
section	2
sources	234
subsection	85
table-of-contents	444
undetermined	5
volume-summary	20

Currently, a number of PA/HO staffers are engaged in a card sort activity to help guide our construction of more discrete FRUS search/filter options.