Build function to factorize to derive multiple geoflow data from data files

[eblondel]

With the need to have unique geoflows that would include processing of a source input, while having the need to associate multiple outputs (resulting from the processing) as multiple layers associated to a single geoflow entity, there is a need to factorize the code at https://github.com/r-geoflow/geoflow/blob/master/R/geoflow_data.R#L520 into a function.

An example of processing geoflow where similar code has been recycled: https://github.com/eblondel/geoflow-geoinfo/blob/main/actions/iotc/process_master.R#L87