df_get_row_by_key(), df_get_row_by_id(),
df_set_row_by_key(), df_set_row_by_id() added
(#25).default_validate_dag() added (#11).Fault tree example added.
Documentation cleaned up.
create_rollup_tree() (new) creates a tree suitable
for rollup() from a data frame with child/parent identifier
pairs.