Support for efficient rational arithmetic

[jdbancal]

Some computations always involve rational representations/bases, for example when decomposing permutation representations of (various products of) symmetric groups.

• We can first use an adhoc Matlab implementation that delegates to double-encoded integers or java.math.BigInteger. See the replab.Rational class in this branch [WIP] Exact arithmetic replab#356.

• SuiteSparse has a rational linear equation system solver with Matlab support, https://github.com/DrTimothyAldenDavis/SuiteSparse/blob/master/SLIP_LU/MATLAB/SLIP_demo.m