correct default settings within geometry_config.py

[anupama-reghunath]

Since within run_simScript.py we always pass the default value, the default settings within geometry_config.py were never used.

[olantwin]

These defaults are for backwards compatibility, to allow e.g. reconstructing old files before helium and removal of the SND were options, so I don't think these need updating.
Instead, we might consider throwing an error when trying to reconstruct these old files (and maybe give information this commit it was generated with, so that it can be reconstructed with the correct version).

[anupama-reghunath]

atleast within the context of DecayVolume, the current version is not backwards compatible. 'vacuums' as default uses the current geometry for vacuum as opposed any of the older geometries with 'vacuums' as decayVolumeMedium.

[olantwin]

Then at least for the DecayVolume we should probably just raise an error (with a helpful error message), instead of constructing an incompatible geometry.

[olantwin]

@anupama-reghunath Any updates on this PR?

[olantwin]

Since we can still construct the SND, its default value for old geometries (which had an SND) should be True. @antonioiuliano2 what do you think?

Looks good to me otherwise.