Bug 568167: [RJ-Data] Change hierarchy of factor stores (RFactorStore no longer extends RIntegerStore) fixing nullable definition of RStore#getChar Follow-up-to: d3633e3402e37b80b615fb2aadbf93b1d3a8dae2 Change-Id: I78375a313a3db909af79b27f36512dcdff66b028