create `download` method for `get_chroncontrol`

[SimonGoring]

Currently if someone wants to get the chroncontrol tables for a set of records they have to do the following (from scratch):

library(neotoma)
nh11k.datasets  <- get_dataset(datasettype='pollen', ageold = 12000,ageyoung=-100,
                               altmin = 100, altmax = 105)
nh11k.downloads <- get_download(nh11k.datasets)

chron.lists <- lapply(nh11k.downloads, function(x)
                                      get_chroncontrol(x$sample.meta$chronology.id[1]))

It would be better if we could just wrap this more clearly into a chroncontrol object, with a method for downloads.

[SimonGoring]

This issue is resolved with commit 2c452017767fb4c65d54ebe4d3d97de6b5816e37. There are now methods for download and download_list objects.