Support for meshed thermal grids

[stroitzsch]

The current implementations in the thermal grid models module cater only to radial networks. This can be extended to cater for meshed networks based on existing approaches from literature.

• [x] Identify branch loops for meshed networks [1].
• [x] Update thermal grid model indexes / incidence matrices to align with [2].
• [ ] Implement Newton-Raphson method for thermal power flow solution based on [2].
• [ ] Revise linear thermal grid model formulation for meshed networks.
• [ ] Document Newton-Raphson method / revised linear grid model.
• [ ] Add test cases for radial / meshed networks.

[1] https://doi.org/10/d5h9sh [2] https://doi.org/10/f4tgrg