Pass extra arguments mice.impute.rf() to ranger::ranger()

[edbonneville]

Hi! This PR allows to pass ... arguments to ranger::ranger() when using mice.impute.rf().

• ... was only being passed along when rfPackage = "randomForest"; should want the same behaviour with the now default rfPackage = "ranger".
• I initially just changed fit to ranger::ranger(x = xobs, y = yobs, num.trees = ntree, ...), but this triggers unused arguments warning for "type" (which I think is passed along to all mice.impute.method functions?). Hence, I use list(...), set extra_ranger_args[["type"]] <- NULL, and proceed with do.call() to avoid this warning.
• Motivated in part because some of the extra ranger() arguments (e.g. mtry) can affect the quality of the imputations quite a bit.

[stefvanbuuren]

Is a useful extension, but please implement in a slightly different way:

• The ranger() function produces a warning for any argument name that is not in its argument list. It does not have a silent option, so also other names produce warnings.
• Use fit <- suppressWarnings(ranger::ranger(x = xobs, y = yobs, num.trees = ntree, ...)) instead. This style is also used in various places in mice.impute.xxx() functions (though it has some downsides).
• We cannot easily get rid off the global type model as this it used in various 2l functions.
• A neater alternative for passing down argument is the blots facility (from "block-dots"), which allows you to specify dots argument for each block (this doesn't however solve the type warning).

Looking forward to your tweak.

[edbonneville]

It has now been edited.. thank you also for mentioning the blots facility, which I was not yet aware of (and is indeed neater!).