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 develop -D org -I include -E exclude: PR 528 (Tile performance improvements for VMS and NDPI)
merge develop -D org -I include -E exclude: PR 857 (Initial support for Zeiss .lms files)
merge develop -D org -I include -E exclude: PR 858 (Initial support for Veeco AFM data)
merge develop -D org -I include -E exclude: PR 859 (Assorted metadata/tile parsing fixes)
merge develop -D org -I include -E exclude: PR 884 (Make a buildable source release)
merge develop -D org -I include -E exclude: PR 885 (Correctly set Modulo annotations for multi-Image datasets)
merge develop -D org -I include -E exclude: PR 886 (Flex: be more permissive in parsing IFDs and XML)
merge develop -D org -I include -E exclude: PR 891 (Fix tiled TIFF reading and writing (rebased onto develop))
merge develop -D org -I include -E exclude: PR 897 (Implement pre-loading for .sdt files in the interest of performance. (rebased onto develop))
merge develop -D org -I include -E exclude: PR 906 (MATLAB toolbox: package and release fixes (rebased onto develop))
merge develop -D org -I include -E exclude: PR 910 (Logback conversion (rebased onto develop))
merge develop -D org -I include -E exclude: PR 926 (Cache OME-XML between ImagePluses)