Write relaxation data to a file.
ri_id: The relaxation data ID string.
file: The name of the file.
dir: The directory name.
bc: A flag which if True will cause the back-calculated data to be written to the file.
force: A flag which if True will cause the file to be overwritten.
If no directory name is given, the file will be placed in the current working directory. The relaxation data ID string is required for selecting which relaxation data to write to file.