Constituent contains a char offset

[danyaljj]

Write the dual method of isConsituentInRange which accepts an offset and decides whether it's included inside the constituent span.

https://github.com/IllinoisCogComp/illinois-cogcomp-nlp/blob/master/core-utilities/src/main/java/edu/illinois/cs/cogcomp/core/datastructures/textannotation/Constituent.java#L471-L473