Update the HelloGeoRead example with the use of the new I/O helpers.

The example now compiles three executables:

1) main1, which uses the loadDB helper to pick the 'world' volume from
   the input .db and loop over its children;

2) main2, which uses the 'getReaderDB' helper to get a reader object,
   with which it reads data rows from the input .db file. It then picks
the 'world' volume from the same .db file with the 'loadDB' helper and
loops over its children;

3) main3, which performs the same operations than main2 but without
   using the new helper functions. It uses the lower-level GMDBManager
and GeoModelRead directly. That shows how to use those to users who
need to perform lower-level operations.