different sampling regimes for different predictors/responses

[joelpick]

Think the best way to do this would be to specify different sampling regimes, and then have a way of combining them, by specifying which variables have which sampling regime.

Could do this in get_sample_data(), something along the lines of giving a list
list(1=c("rain"), 2=c("y"))
which would indicate that the first sampling regime was used for "rain" and the second for "y". This would then enable different response variables to be sampled differently, for example
list(1=c("y1"), 2=c("y2"))

It would also be a good idea to allow the sampled data to be returned with NAs in it, for example, have an argument include_NA=TRUE (with FALSE as default)