Use hydra for args and defaults

[claforte]

The way we handle arguments and override defaults is messy and confusing. I think it might be cleaner to use hydra instead, similarly to https://github.com/threestudio-project/threestudio.

I might do it myself soon since the defaults overridden here-and-there are hindering my progress. ;-)

@ashawkey what do you think?

[csawtelle]

Sorry to pop in here. This would be really helpful!

[ashawkey]

I agree. The current option system is quite confusing indeed... If you could help with this it would be really appriciated!