PrairieReader: purge broken links, add author tags
PrairieMetadata: fix typo in javadoc
PrairieReader: consolidate XML/CFG references

Rather than having separate hardcoded methods, let's just have one
method called "findMetadataFiles" that looks for all the unspecified
ones. And refer to them as "Prairie metadata files" for generality.

This will be useful when we add support for ENV files shortly.
PrairieMetadata: tweak flow of channel parsing

We should at least _try_ to parse the active channels, even if a CFG
file was not present. This will be useful soon, when that information
could potentially come from another source (the ENV file).
PrairieReader: also parse ENV metadata files

Start with version 5.2, the CFG file was replaced with an ENV
that is similar conceptually, but with a revised XML schema.

It does contain many of the same key/value pairs, though, including
the ones in which we are most interested (channels, bitDepth, laserPower
and xYStageYPositionIncreasesBottomToTop in particular), so for now we
parse the metadata into the same config table used by the CFG file.