Added coupling between pySDC and Gusto (#516)

* Added coupling between pySDC and Gusto

* Removed unused parameters

* Update pySDC/implementations/problem_classes/GenericGusto.py

Co-authored-by: Dr Jemma Shipton <j.shipton@exeter.ac.uk>

* Update pySDC/implementations/problem_classes/GenericGusto.py

Co-authored-by: Dr Jemma Shipton <j.shipton@exeter.ac.uk>

* Update pySDC/implementations/problem_classes/GenericGusto.py

Co-authored-by: Dr Jemma Shipton <j.shipton@exeter.ac.uk>

* Update pySDC/tutorial/step_7/F_pySDC_with_Gusto.py

Co-authored-by: Dr Jemma Shipton <j.shipton@exeter.ac.uk>

* Minor refactor

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Co-authored-by: Dr Jemma Shipton <j.shipton@exeter.ac.uk>