Feature Request: Interpolate for Mean/ ETS

[yijunwang0805]

Hi Mitchell!

I notice interpolate does not support MEAN or ETS models.

library(tsibbledata)
data("olympic_running")
olympic_running %>%
  model(MEAN(time)) %>%
  interpolate(olympic_running)

Error: Column `time` is of unsupported type function
In addition: There were 29 warnings (use warnings() to see them)

Is it possible to add these?

[mitchelloharawild]

ETS does not support missing values, but I can certainly add support for MEAN.