Add option to set individual cut offs for each participant

[milanwiedemann]

As raised by a reviewer, it might be of interest to set individual cut offs for each participant. This would be similar to the modification of the third criterion used by Kelly et al 2005 and 2007 to assess stability.

This could be implemented in the R package as a modification of the first or third criterion. Computationally this approach is similar to the way the first criterion gets assessed.
Conceptually the way individual cut off scores are calculated would is closer to third criterion (stability). There could also be other useful applications of individual cut off scores calculated in a different way to Kelly et al.

Kelly, M. A. R., Cyranowski, J. M., & Frank, E. (2007). Sudden gains in interpersonal psychotherapy for depression. Behaviour Research and Therapy, 45(11), 2563–2572. https://doi.org/10.1016/j.brat.2007.07.007

Kelly, M. A. R., Roberts, J. E., & Ciesla, J. A. (2005). Sudden gains in cognitive behavioral treatment for depression: When do they occur and do they matter? Behaviour Research and Therapy, 43(6), 703–714. https://doi.org/10.1016/j.brat.2004.06.002