[PLAT-6613] Added description (definition and derivative) for Bill TRS.
[PLAT-6613] Added pricing method for Bill TRS.
PLAT-6625 - Bill TRS security/converter and model integration + legal entity utils
PLAT-6613 cleaned up legal entity utils
[PLAT-6613] Improve javadoc on Bon and Bill TRS. Change Bill TRS converter to use a notional of 1 for underlying. (Ian)