This small PR adds the option to sort loci when creating a PoleZeroData object. This is useful if you create a PoleZeroData object in some way other than calling root_locus_map (which sorts the data ahead of time).

The need for this functionality came up when working through Example 5.17 of FBS2e (bicycle eigenvalue location versus speed), as shown in the following code:

https://github.com/murrayrm/fbs2e-python/blob/main/example-5.17-bicycle_stability.py

(At some point I would like to generalized the root locus capability to allow plotting an eigenvalue locus diagram against any free parameter in a nonlinear system, but that will take more time and effort.)