Move essentially all the content of procedure get_snapshot to main

unit test_get_snapshot. get_snapshot reads from shapefiles instead of
netCDF files.

Bug fix: programs test_get_snapshot and test_read_snapshot do not need
dispatch_snapshot.f and send_snapshot.f.

Procedures dispatch_snapshot, get_snapshot, send_snapshot and
receive_snapshot are no longer referenced, for now. They will be
called by the overlap program.