PrairieMetadata: fix critical errors

This change fixes several metadata parsing bugs:

1) The relevant XML element is called "SubindexedValue", not
   "SubindexValue". Without this change, subindexed values are never
   read correctly, so e.g. X/Y/Z stage positions are not parsed.

2) The loop variable for subindexed values is "s", not "i".
   Without this change, an NPE is typically thrown during the
   SubindexedValues parsing loop.

3) Subindexed value table should be indexed on subindex, not index.

4) Parsing of subindexed values from old-style XML (<Key> elements)
   did not add the parsed subindexed value table to the data structure.
Add unit tests for PrairieMetadata

These tests are by no means complete, but do provide regression tests
for the bugs fixed in the previous commit.
WIP