Resolve "Implement new propagator operations for electrons and modify propagators to use it."
Create a propagator_ops class for electrons that provides all the routines necessary to perform propagations for the various propagators.
- Forbidding to use QOCT and CN with gauge field as they do not support it.
- Fusing exponential and density calculation for ETRS and EXP_MID.
- Split the AETRS and CAETRS propagators into two routines, as there is no code to reuse
- Add the missing extrapolation of the vtau for MGGA with exponential midpoint rule.
- Fix a missing allocation/deallocation for CAETRS with MGGAs.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.
Closes #216 (closed)