Fixed NPE in FormatTools.getFilename

The ome.scifio FormatTools was mistakenly querying axes instead of
channels. Fixed this to look through channelDimTypes.

Note that this is not perfectly analogous to the legacy behavior because
of lacking a MetadataRetrieve with individual channel labels. Presumably
if an OMEMetadata was translated to the target Metadata though, the
  channel names would be preserved.