Store information of one keyword or category from the keywords list.

This record stores the information of one keyword or category in the keywlist.txt as a Python dictionary. The keys in this dictionary are the line codes that can appear in the keywlist.txt file::

---------  ---------------------------     ----------------------
Line code  Content                         Occurrence in an entry
---------  ---------------------------     ----------------------
ID         Identifier (keyword)            Once; starts a keyword entry
IC         Identifier (category)           Once; starts a category entry
AC         Accession (KW-xxxx)             Once
DE         Definition                      Once or more
SY         Synonyms                        Optional; once or more
GO         Gene ontology (GO) mapping      Optional; once or more
HI         Hierarchy                       Optional; once or more
WW         Relevant WWW site               Optional; once or more
CA         Category                        Once per keyword entry; absent
                                           in category entries