Add RecommendedDisplayCIELabValue parameter to Annotation Group

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Additionally add color conversion class following implementation here: https://www.dclunie.com/pixelmed/software/javadoc/com/pixelmed/utils/ColorUtilities.html

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Can also use this implementation to compare: https://github.com/DCMTK/dcmtk/blob/master/dcmiod/libsrc/cielabutil.cc

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Addressed in #374