prepare_abcd() runs the necessary steps to prepare the input data sets for
the PACTA for Supervisors analysis. Specifically it prepares the abcd_final
data set by removing inactive companies if desired. And it allows preparing
the ratios by which the exposures to counterparties are split along the sectors.
Parameters for both steps are read from a config.yml file. The function is
called for its side effects and writes the prepared data sets to the directory
output/prepare, where output is the output directory specified in the
config.yml.