refactor

[typoverflow]

Hi Haotian, I drafted this pull request to better illustrate the structural changes. I think there are some things we can do:

• 1. extract a package, spectralrl, to manage the codes and to distribute the codes conveniently with pypi.
• 2. use hydra for argument parsing, instead of argparse, since different algorithms require different sets of arguments
• 3. refactor those image-based algorithms (namely MuLVRep, DiffSR and DrQv2), so that they share the same pipeline and just differ in the implementation of the agents
• 4. refactor the codes for state-based algorithms.

Currently, I have partially completed 1-3 (see spectral/ and examples/main_visual.py). I am still testing the image-based algorithms to make sure they behave in the same way as the original implementations. Could you help with refactoring the state-based algorithms if possible?