pytket v1.6.0 release candidate 2

ToffoliBox Synthesis (#494)

* Add first ToffoliBox implementation

* add toffolibox type with minor optimisations

* further improvements

* Update Boxes.cpp

* fix compilation issues

* Fix compilation issues

* Compilation working suitably, with iterators retrieved once

* issues with merging cycles

* Mkae inimum version

* expose to pytket

* black formatting

* update how transpositions found

* add comments with matrices

* Fix issues

* Update circuit_test.py

* Update Boxes.hpp

* Make changes in line with review

* Update Boxes.hpp

* Change to copy constructor

* format

* Update changelog.rst