a sequence of unique station Ids. Ids start at 99111111. They
are present as variables in both the file names and the inventories.
Ids defaults to NULL. If this is not changed by the caller then all
station Ids are used to create the dataset. Alternatively, one can create
a subset of all the data by subseting the inventory and working with the
Ids from that inventory. For example, one could create datasets for every
province or for lat/lon combinations.
The directory defaults to EnvCanada where all the csv files
are. The csv files all have unique names that are tied to the unique Ids.
Given the vector of Ids provided by the caller, and the list of files
available, the function then reads in those files to generate a data structure.