Add `dpgui` entry point and `dpgen2 gui` CLI

[njzjz]

See https://github.com/deepmodeling/dpgui/issues/270 for details.

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (9ab5a87) 88.12% compared to head (2bfb2c2) 87.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #172 +/- ## ========================================== - Coverage 88.12% 87.98% -0.14% ========================================== Files 80 81 +1 Lines 4032 4045 +13 ========================================== + Hits 3553 3559 +6 - Misses 479 486 +7 ``` | [Files](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpgen2/entrypoint/main.py](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBnZW4yL2VudHJ5cG9pbnQvbWFpbi5weQ==) | `54.86% <66.66%> (+0.66%)` | :arrow_up: | | [dpgen2/entrypoint/gui.py](https://app.codecov.io/gh/deepmodeling/dpgen2/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBnZW4yL2VudHJ5cG9pbnQvZ3VpLnB5) | `28.57% <28.57%> (ø)` | |

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