Generic support for additional metadata

[jefferis]

The PR for additional fanc_meta (#22) gives one pattern, where we just return accept a path to a file on disk (tsv/feather) or a dataframe. I think more generically we could provide the option to register a metadata function (possibly even more than one) for a dataset.

cf_register_meta<-function(dataset, FUN, add = TRUE, after = TRUE) {

}

Where FUN would look like

FUN<-function(ids=NULL, ...) {
  # fetch metadata
  return(df)
}

using cf_register_meta would allow the actual method of storing the metadata FUN to evolve as we change coconatfly. Furthermore it would be a step on the way to actually using the coconat::register_dataset function (which could perhaps use this under the hood?).

Questions/todos

• how do we handle registering of duplicate functions?
• What does add=FALSE mean (does it completely replace the built-in function)?
• What does after mean (does it take priority over the functions that precede it (probably, yes)?
• Do we allow both dataframe or file input?
• Who handles updates for CAVE type data? Probably coconatfly since this is quite generic, but then we should provide a supervoxel_id column to signal this.
  • coconatfly would also need to figure out how to do this update for the specified dataset.