Fix for single channel  meas_mode 13 data. As example provided by david @Montpellier
Merge branch 'dev_5_0' of https://github.com/openmicroscopy/bioformats into dev_5_0_measurementMode13
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.
InCell: fix NPE when parsing Z positions

See QA 7987 and
http://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7474
Merge pull request #960 from melissalinkert/rebased/dev_5_0/cache-ome-xml

Cache OME-XML between ImagePluses (rebased onto dev_5_0)
Merge pull request #988 from melissalinkert/incell-npe

InCell: fix NPE when parsing Z positions
Merge pull request #915 from imunro/dev_5_0_measurementMode13

Fix for single channel  meas_mode 13 data.