implement more OPs and workflows

[njzjz]

DPGEN2 refactor DP-GEN based on dflow. The technical details can be found here. Currently, we have only implemented DP+LAMMPS+VASP workflow.

The following features, which are part of DP-GEN, have not been implemented into DPGEN2 yet:

• Workflows
  • simplify
  • init data
• MD OPs
  • Gromacs
  • AMBER
  • calypso
  • extra features like enhanced sampling
• FP OPs
  • cp2k
  • gaussian
  • siesta
  • abacus
• Taking clusters for sampling

DPGEN2 also accepts features that were not implemented in DP-GEN.

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Notes:

1. Unit tests and examples should be added.
2. PEP-8 should be obeyed.
3. DP-GEN codes can be reused under the LGPL-3.0 license.
4. DPGEN2 offers some useful utils, including dpgen2.utils.chdir.set_directory and dpgen2.utils.run_command.run_command.

[njzjz]

DeepModeling Hackathon 2.0 task

Implement 3 new OPs or workflows as mentioned above. Successfully run examples through the command interface (CLI) with new OPs or workflows. Open pull requests to submit new OPs, CLI, unit tests, and examples.

[zjgemi]

#238