Fixing some bugs, updating the docs and requirements

[WenjieDu]

What does this PR do?

1. fixing Inability to Utilize Specific CUDA Devices for Training in Raindrop Model #304, Unexpected LOCF object initialization #306, and Testing on GitHub times out because of blocked plotting functions #309;
2. remove the requirement limitation on pandas<2.0.0;
3. update the docs;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 8292683604

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 81.654%

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Totals
Change from base Build 8261646531:	0.02%
Covered Lines:	5123
Relevant Lines:	6274

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💛 - Coveralls