Merge DeepH-E3 and xDeepH into the current repository

[mzjb]

Merge two methods, DeepH-E3 and xDeepH, into DeepH-pack. This will allow us to better integrate the DeepH series methods and maintain them more effectively.

Here are the details of the two methods:

1. DeepH-E3

• Repository: https://github.com/Xiaoxun-Gong/DeepH-E3
• Paper: General framework for E(3)-equivariant neural network representation of density functional theory Hamiltonian
  (accepted by Nature Communications)

2. xDeepH

• Repository: https://github.com/mzjb/xDeepH
• Paper: Deep-learning electronic-structure calculation of magnetic superstructures (accepted by Nature Computational Science)