Fixed leak in Mutator object
Fixed leak in Mutator object

Added two tests to prevent self._axes to fill up with dupes of the same
axis. This could result in better performance if the mutator object is
used to generate multiple instances. Thanks to
https://github.com/loicsander for the analysis and graph.

LettError/MutatorMath,miguelsousa/MutatorMath,moyogo/mutatormath,anthrotype/MutatorMath