Generalise external scans

[amereghe]

Port external scans to generate new studies not only based on .mask files, but also on other input files -- actually, sixdeskenv, sysenv and fort.3.local.

This feature breaks the difference internal vs external scans:

• internal: from this point on, classical scans handled via sixdeskenv;
• external: all the rest.

At the same time:

• add echo of generated studies;
• ask user to list files where placeholders are expected to be found via parameters like scan_files_<>, similar to scan_values_<>. If no file is specified, only .mask is considered;