C++: Add FormatWriter interface

Initial port of `IFormatWriter`.  Note needs the questions in http://lists.openmicroscopy.org.uk/pipermail/ome-devel/2014-September/002944.html addressing.  Some have been partly addressed by omitting certain methods for the time being.

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Testing: It's a header, so nothing to test.  The header and documentation checks will make sure it's valid code and has all the docstrings.