Specify median for defSurv and genSurv and/or add Weibull distribution

[kgoldfeld]

survival data is generated in simstudy with an underlying Weibull distribution, which is specified with a shape and scale parameter. A user has suggested being allowed to specify the median (given a specific shape (lambda) or scale (k) parameter, but not both):

We could even go further and allow the user to specify the mean and allow the mean to be a function of predictors, just as we do with other distributions.

Or we could just add the weibull distribution to the list of possible distributions in genData.

[assignUser]

How important/useful is this to you? Would you like this in the next release?

[kgoldfeld]

Not super important for the next release, no.

[assignUser]

I think this will also be easier to solve with #75