Add a class diagram exporter

[Wuestengecko]

This is effectively the inverse operation of the existing json2capella, i.e. capella2json.

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• The extends field shows an empty string instead of the class it is extending.
• When I manually input the description fields from scratch, it includes tags <p>...</p>\n in the info field.
• The dataType or enumType field is not present for the attributes.
• The composition or reference field is not present for the associations.