Perform the actual lookup for a key. The keytype and targettype can be considered as field names in a typical
relation. doLookup find the target value that corresponds with the keyvalue that appears in the keytype field
of a record. Returns the first one that matches if there are muliple occurrences of the keyvalue.
keytype - Actually the CharGer type for what a particular key is; e.g., "Birth Date" or "Gender"
keyvalue - The CharGer referent value for a concept of type keytype
targettype - the CharGer type for the key value to be looked up, as in keytype
target value that corresponds to the targettype associated with the keytype and value, null if
there isn't one.