average_toxicity: Average Toxicity Number
In dnzmarcio/ewoc: Escalation with Overdose Control

Calculate the Average Toxicity Number.

1	average_toxicity(dose, dose_set, true_prob, theta)

`dose`	a numerical matrix of assigned doses for each step of the trial (column) and for each trial (row).
`dose_set`	a numerical vector of allowable doses in the trial.
`true_prob`	a numerical vector of the true probabilities associated with 'dose_set'.
`theta`	a numerical value defining the proportion of expected patients to experience a medically unacceptable, dose-limiting toxicity (DLT) if administered the MTD.