ImportProcess: add lazy getOMEXML() accessor

This accessor extracts the OME-XML string from the OME metadata, caching
it if it is later accessed again. In this way, the same ImportProcess
will always return the same OME-XML string.
ImagePlusReader: get OME-XML from ImportProcess

The ImportProcess takes care of caching the OME-XML string so that we
don't have to generate a new copy of the OME-XML every time we need one.

This change reduces memory use when BF.openImagePlus needs to create
many ImagePlus objects. Previously, every ImagePlus object would have a
separate copy of the OME-XML embedded in its FileInfo#description field.
With this change, each description field references the same string.
Merge pull request #926 from ctrueden/cache-ome-xml

Cache OME-XML between ImagePluses