Fixed ImageMetadata population

When populating ImageMetadata using an OMEXMLMetadata, was accidentally
counting size Z as both Z and T axis lengths. Fixed.
Changed default calibration values

ImgOpener should now populate default calibration values of 1.0 instead
of NaN, to allow computations that assume calibration values to be
performed.
WIP: ImgSaver

Updated signatures to prevent infinite loop/NPE

Also force translation to SCIFIO Metadata, to trigger format-specific
translation processees.