added enr_ feature

[GyunghunK]

Added the energy restricted space (enr) feature to jaxquantum. The code structure follows the QuTip implementation of the same feature.

[Phionx]

Thanks for the PR @GyunghunK ! Please pull the new changes from the main branch in this repo to your branch, so that this can be merged in smoothly. Then, you will not be any number of commits behind EQuS/jaxquantum:main, as shown in the screenshot. You can do this manually via the git CLI or you can just press the "3 commits behind" text in the screenshot and it will automatically create a PR that pulls in recent changes from EQuS/jaxquantum:main into your branch. Let me know if you have any questions on how to do that.

[GyunghunK]

Happy new year Shantanu! I have merged the main branch to this one. Please check it out!

[Phionx]

Great @GyunghunK , thanks for the quick update and happy new year to you too!

The code itself looks good to me.

Before I merge this, could you please add to the docs in two spots?

1. Motivation and example usage of enr: https://github.com/EQuS/jaxquantum/blob/main/docs/advanced/enr.md
   Please expand on this computational technique a bit more than in the jupyter notebook so that it's clear when the ENR basis is worth using. If you can, a runtime or memory simulation would be awesome (and very useful!).

2. Docs on how to run tests:
   https://github.com/EQuS/jaxquantum/blob/main/docs/getting_started/tests.md
   Please add a note on how to locally run the tests you added.

You may have to merge main into your branch again before you see these files.

[Phionx]

Hey @GyunghunK , can you please merge main into this branch and check if your tests still pass? If so, please let me know and I'll merge it. If you can add the motivating example I mentioned in my last comment, that would be great. Even if not, I'd like to merge this in and close this PR. Thanks!

[GyunghunK]

Sorry for the delay. I have merged the main and checked the notebook and tests regarding the enr submodule. Thank you!