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.

[jefferis]

Maybe we should start a bit simpler and just allow one metadata function per dataset. However it would probably be appreciated a) if we have a mechanism to unset (set NULL?). Also coconatfly should preferably handle processing of the URLs. This should possibly include the ability to download the relevant git repo as in the original fanc_meta option.