Scope-rl

Upgrade d3rlpy version

- Previously, SCOPE-RL supported compatibility with `d3rlpy==1.1.1`. **From `scope-rl==0.2.1`, we support compatibility with `d3rlpy==2.0.4`.**
- There are no user-side interface changes on SCOPE-RL.
- We update quickstart examples accordingly.
- Some minor fixes on policy modules and visualization functions.

**References**
- PR 18
- Issue 17

An emergency patch to deal with some bugs in plot functions and policy heads used in continuous action environments.

- Some fixes on the plotting function implemented in the OffPolicySelection module. (Escaping `KeyError: y` in the swarmplot.)
- Some fixes on the `GaussianHead` and `TruncatedGaussianHead`

An emergency patch to deal with recent interface changes of [d3rlpy](https://github.com/takuseno/d3rlpy). We will work on upgrading scope-rl to adapt to the changes of d3rlpy in future releases.

- Related issue 17

The initial release of the package 🎉

- documentation: https://scope-rl.readthedocs.io/en/latest/
- PyPI: https://pypi.org/project/scope-rl/