Add self-describing header to compute_shc output (shc.out)

[erhart1]

This PR adds a #-comment header block to shc.out, following the pattern established for other measure output files (#1579).

shc.out contains two structurally distinct datasets (correlation function and spectral heat current) with different column semantics, so the header uses # columns_correlation and # columns_shc instead of a single # columns line.
The header also records # num_correlation_rows and # num_frequency_rows to make the block boundaries unambiguous for parsers.
When group_id -1 is used (all groups), # num_output_groups gives the number of block pairs written.