FileLoader.cpp: create hash and link

Local resources (material map, calibration files, field maps) have:

• fixed local file name (e.g. config/material-map.cbor),
• source location URL (e.g. https://eicweb.phy.anl.gov/EIC/detectors/athena/uploads/4a4e7c8eb6089b634d762d112c89bd5d/material-maps.cbor with hex hash 8b21c068916be3c4),
• optional cached local copy (e.g. /opt/detectors/resources/8b21c068916be3c4).

An updated source location URL should recreate local filename link (i.e. link should always be recreated if there is a chance it points to the wrong things).

A local configuration with custom files should no be modified (potentially limited by the fact that the local configuration should not use symlinks). No actual files should ever be deleted; at most links should be modified.

This will allow for clearer treatment of resources:

• create hash from URL,
• search for hash in cache,
  • if found: link hash to cache,
  • else: download from URL to hash,
• link local file to hash.