arrow version: support colnames()

[schuemie]

The new arrow version does not support colnames(). colnames() is not a generic, but it calls dimnames, which is a generic. Adding this code to Andromeda will add support for colnames():

dimnames.ArrowObject <- function(x, do.NULL = TRUE, prefix = "col") {
  return(list(NULL, names(x)))
}

[ablack3]

implemented in develop branch