New features:
* Programmatically add gates, transforms, and compensation matrices to a GatingStrategy
* Export a GatingML 2.0 compliant document from a GatingStrategy
API improvements:
* Gate sub-classes no longer require a GatingStrategy in their constructor, allowing re-use of instantiated gates
* New convenience methods in the Session class for getting parent gates and creating gates, transforms, and comp matrices
* Better x & y axis labels in scatter plot (includes marker names, if available)
Bug fixes:
* Fixed bug in GatingStrategy method get_gate_by_reference where an existing QuadrantGate could not be found
* Fixed bug in parsing spillover matrix from a $SPILL keyword value of an FCS file