StoppingMTDdistribution-class: Stop based on MTD distribution
In 0liver0815/onc-crmpack-test: Object-Oriented Implementation of CRM Designs

Has 90% probability above a threshold of 50% of the current MTD been reached? This class is used for this question.

target: the target toxicity probability (e.g. 0.33) defining the MTD
thresh: the threshold relative to the MTD (e.g. 0.5)
prob: required probability (e.g. 0.9)

# As example, here is the rule for: 
#   stopping the study if there is at least 0.9 probability that MTD > 0.5*next_dose.
#   Here MTD is defined as the dose for which prob(DLE)=0.33

myStopping <- StoppingMTDdistribution(target = 0.33,
                                      thresh = 0.5,
                                      prob = 0.9)