DOC: A bit of documentation.

.gitlab-ci.yml

src/main/java/org/caosdb/datetime/FragmentDateTime.java

src/main/java/org/caosdb/datetime/UTCDateTime.java

...in/java/org/caosdb/server/datatype/AbstractEnumValue.java

...main/java/org/caosdb/server/datatype/CollectionValue.java

src/main/java/org/caosdb/server/datatype/ReferenceValue.java