Missing support for YCbCr601

[hbrodin]

Hi,
We have encountered a file that causes Nitro to abort in debug mode.
It seems like the IREP header field parsing does not properly support YCbCr601.

The parsing of string value to enum member fails as the corresponding string is not in the map generated from the below code.

nitro/modules/c++/nitf/include/nitf/ImageSubheader.hpp

Lines 68 to 72 in b39ccc4

	enum class ImageRepresentation { MONO, RGB, RGB_LUT, MULTI, NODISPLY };
	NITF_ENUM_define_string_to_enum_begin(ImageRepresentation) // need to do this manually because of "RGB/LUT"
	{ "MONO", ImageRepresentation::MONO }, { "RGB", ImageRepresentation::RGB }, { "RGB/LUT", ImageRepresentation::RGB_LUT }, { "MULTI", ImageRepresentation::MULTI },
	{ "NODISPLY", ImageRepresentation::NODISPLY }
	NITF_ENUM_define_string_to_end

Affected version is run on ubuntu, built from from commit b39ccc4