Improving the GeoModelIO 'saveToDB' helper function

This MR adds the forceDelete option to the GeoModelIO saveToDB helper function, which was suggested by @todorova in !217 (comment 7238241)

The option is false by default.

If false and if a file with the same name is found in the run folder, GeoModelIO by default prints a message and throws an exception.

By setting it to true, the user can ask GeoModelIO to replace an existing .db file.

This MR also updates the GeoModelIO GMTests_IO.h, which uses the helper function to perform different tests on an input GeoVPhysVol.

This MR also updates the examples in GeoModelExamples that use the helper function, notably, the tests of shared nodes.

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This MR relates to #62 (closed) and closes #96 (closed)