Fix writing of tiff description

toCharArray() is the same as getBytes('UTF-16BE')
This leads to have unicode for units not being
correctly encoded.
Add test checking that the reader used is the OME-TIFF reader.
Add doc.
cmake: Replace FindXerces with FindXercesC

Upstream renamed the macro to avoid confusion with the
Java implementation.
cpp: bioformats: Correct placement of warning pragmas in PixelProperties
Merge pull request #1459 from jburel/encoding

Fix writing of tiff description
Merge pull request #1467 from rleigh-dundee/cmake-xerces

cmake: Replace FindXerces with FindXercesC
Merge pull request #1469 from rleigh-dundee/cpp-pixelprop-warnings

cpp: bioformats: Correct placement of warning pragmas in PixelProperties