Replace ERA20C (retired) by ERA5 dataset

[hweifluids]

Dear Maintainers,

I hope this message finds you well.

The dataset currently demonstrated in tutorials-climate (ERC20C) was officially announced to be retired, and API could no longer be called, therefore these tutorials cannot run properly any more (both 2D and 3D). The newly-released open-licensed dataset ERA5, coming with new API but has almost the same dataset structure, is used instead.

More small bugs have been fixed.

Wish you all the best.

Huanxia Wei