xsd-fu: C++ OMEModelObject template avoids DOM text content rvalue references

Needed to build with clang++ on FreeBSD, and fixes a type conversion error.
cmake: Add extra-warnings option
cpp: bioformats: SaveSeries can not throw on destruction
cpp: bioformats: Correct currentId assertions in FormatReader
cpp: Add test component

The test component produces an ome-test static library which all tests
should link against.  This depends upon gtest and provides headers which
tests may use.  This consolidates the test dependencies into a single
place, and allows the provision of common test facilities which may be
used by all tests.
cpp: test: xerces tests use ome-test and gtest
cpp: ome-xml: Add ModelException

The Java implementation throws a RuntimeException, which was being
mapped to a C++ std::runtime_error.  This adds a ModelException
class derived from std::runtime_error so it's possible to distinguish
model errors from other generic errors.
xsd-fu: cpp: Make model object arguments more specific
xsd-fu: cpp: Remove todo items

These are items which are either not valid or are not doable.
xsd-fu: cpp: Remove additional todo items
cpp: ome-xml: OMEModelObject compare_element is strictly const
cpp: bioformats: Correct description of UnsupportedCompressionException
cpp: Document detail namespaces
cpp: bioformats: Replace image_size_type with dimension_size_type

image_size_type was redundant since the image indexing is a
combination of the dimensions all using dimension_size_type.
Using dimension_size_type for everything is simpler and more
consistent.
cpp: bioformats: Correct exception message in FormatHandler
cpp: bioformats: Document absence of CoreMetadata construction from reader
cpp: bioformats: FormatReader::isUsedFile does not throw boost::filesystem::filesystem_error
cpp: bioformats: CoreMetadata sets bitsPerPixel to 0

This makes it default to the pixel type size, or else it
would be inconsistent.
cpp: bioformats: Add CoreMetadata and Modulo stream operators
cpp: bioformats: Correct used files signatures in FormatReader