write_run_data: Write data into the run directory
Description
Provide a function that will write data into the active run
directory (if and when it's avaialble).
Usage
write_run_data(name, write_fn)
Arguments
name
Name of data to write (subsequent writes with the same name
will overwrite).
write_fn
Function that writes the data. The function will be
passed a single run_dir argument.