Helper to load transfer entropy results from a combined CSV file or from separate per-type files (counts, rates, binary) if the combined file is not available.
.read_te_all(dir_csv)A data frame with transfer entropy results (potentially
combining several types) or NULL if no files are found.
Character scalar; directory where the transfer entropy CSV files are stored.
The function first looks for "transfer_entropy.csv". If present,
it is read and returned. Otherwise it attempts to read:
"transfer_entropy_counts.csv"
"transfer_entropy_rates.csv"
"transfer_entropy_binary.csv"
and combines them into a single data frame with a type column.