Write the PCS data to file.
align_id: The alignment ID string.
file: The name of the file.
dir: The directory name.
bc: A flag which if set will write out the back-calculated rather than measured RDCs.
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 alignment ID is required for selecting which PCS data set will be written to file.