Make tolerance overridable

[hannobraun]

Fornjot works with a tolerance value, that specifies how far a generated triangle mesh is allowed to deviate from the ideal geometry that a model defines. Right now, this value is computed automatically, based on the total size of the model. This is fine as a fallback, but the user should be able to override this value.

A command-line argument can be added in args.rs and then processed in main.rs. As of this writing, the tolerance is also computed in main.rs, so integrating an argument there should be straight-forward. Labeling as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, as this is a small change that doesn't require much knowledge of Fornjot.

[mxdamien]

Hey ! :)

See #352 for a suggestion.