Implement the complete electrode model (CEM)

[liubenyuan]

Complete electrode model (CEM) could better modeling the current distributions on the electrode and the contact impedances, CEM should be incorporate into pyeit.

• block matrix calculation of the stiffness matrix.
• how to modeling the CEM into the PyEITMesh class? PyEITProtocol class could remain the same.
• Forward computing of CEM and visualization. Compare the results to the simple electrode model.

[liubenyuan]

Existing projects using CEM: EIDORS, and

1. https://github.com/NablaIP/Scrambled-Eggs/blob/main/Scrambled_Eggs.ipynb
2. https://github.com/ivo53/EIT-MPhys-Project, https://github.com/fbbarnes/eitVISA
3. https://github.com/fsmMLK/openSAHE