ImgSaver#populateMeta populates physicalPixelSizes

The populateMeta method now calls the IMetadata setPixelsPhysicalSize
X/Y/Z methods, using the Img's calibration.

This should allow calibration data to be preserved between I/O
operations.
Merge pull request #12 from hinerm/physical-pixel-pop

PopulateMeta populates physicalPixelSizes