testing Monte Carlo integration

[nhejazi]

The function https://github.com/nhejazi/medshift/blob/continuous/R/utils.R#L235-L289 for Monte Carlo integration in the case of continuous A may not require re-scaling by the range of the uniform distribution (i.e., no need to use the multiplier b - a) notes @jeremyrcoyle. This should be tested --- a simple case would be to let the density mechanism be a normal distribution and the weighting mechanism to be the identity.