modify ConstantNumberGenerator, generalize to ConstantGenerator (enforcing numbers not necessary)
add ZipfianNumberGenerator, generic version of the YCSB ZipfianGenerator, which only returned Long
add beginnings of GrowingRangeExponentialNumberGenerator (only test class so far)
update package naming to reflect that the project is no longer YCSB, but an LDBC product
add more tests for WorkloadUtilsTest class