Reject stale site indexes

[thierrygosselin]

Summary

When a text sites file is newer than its cached .bin or .idx files, ANGSD previously printed a warning but continued using the stale indexes.

This can cause the -sites workflow to read outdated coordinates or filters, producing confusing downstream results. It is related to the stale-index and zero-retained-site reports in issues #380 and #385.

This change:

• reports the condition as an error;
• stops before reading stale site indexes;
• returns a failure status;
• instructs the user to regenerate the site indexes.

No site-selection logic is changed when the text file and its indexes are up to date.