WordVectorTable should reference WVs by String

WordVectorsTable API retrieves WordVector via CharSequence , this is suboptimal as implementors could store such WVs via an hash table (e.g. MapWordVectorsTable) and the value of CharSequence#toString is not guaranteed to be the stable.
Additionally it's more common to have words as Strings rather than CharSequences, being that more consistent with other OpenNLP APIs (e.g. Tokenizer ).

So WordVectorsTable should instead retrieve {{WordVector}}s using String.