getCtryNlDataFname: Check if a month number is valid for a given nightlight type
Description
Get the name of the data file. This function can be altered to name the file as
required and consistently retrieve the name. Used in the function getCtryNlDataFnamePath
to concat the directory path and this filename. Currently all nlTypes are stored in one
file. Can be altered to separate VIIRS and OLS data files for example.