PrairieMetadata: add API to access wavelengths
PrairieReader: populate emission wavelengths

The Prairie XML records the channel emission wavelengths, so
let's populate the OME-XML with that information as appropriate.
PrairieMetadata: activate all used channels

This makes spectral PrairieView datasets work as expected.

Previously, the reader only activated channels specifically flagged as
active in the Prairie configuration metadata. This was fragile for two
reasons:

1) The Prairie configuration metadata is subject to change
   (and in fact did change once already, from CFG to ENV).

2) PrairieView 5.2 added support for spectral datasets, where the active
   channels metadata no longer accurately reflects the actually active
   channels. Instead, the active channels are meant to be gleaned from
   the "channel" attribute of each <File> element present in the XML
   metadata.

With this change, all channels that are actually used (i.e., which
actually have one or more associated <File> elements) will affect the
final dimensional structure of each Image.
PrairieMetadata: add API to query spectral mode