Implement Default::default() for arrsac::Config

[jerry73204]

I see the suggested parameters on Config doc. It would be reasonable to implement Default trait for Config. That would give us some convenience to create this struct.

[vadixidav]

@jerry73204 The inlier threshold is specific to your dataset and should be provided as a parameter to new. I will leave this ticket open though as that should be added to the documentation.

[jerry73204]

EDIT: I missed the new() method there, and yes it's more like a doc issue.