ImageMetadata: convert to CalibratedAxis

Refactored the ImageMetadata class to use CalibratedAxis introduced in
imglib2-beta10 (updated to pom-scijava 1.65) instead of just AxisType.
This allows SCIFIO to track physical calibration information.

Added API to ImageMetadata, Metadata and FormatTools for working with
calibration values.

Updated all use cases for calibration.

Note that ImgSaver and ImgOpener can now take advantage of calibration
information.