Create function to return definitions file for a station

[ChrisMarsh82]

Station definitions will need to be able to be returned via an API endpoint.

Country and station id should be passed in.

[lilyclements]

PR #54 creates an R Function which returns metadata for stations and the definitions. @ChrisMarsh82 is this suitable here, or would you rather have a function which returns the definitions and station_id, but not the other station metadata?

If the latter, the get_definitions_data in the epicsadata package reads in the definitions. I can call this in here, and return the definitions as a data frame instead of a list. Let me know what format you'd like it in.