Resolve "Implement new propagator operations for electrons and modify propagators to use it."

Description

Create a propagator_ops class for electrons that provides all the routines necessary to perform propagations for the various propagators.

Improvement:

• 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

Bugfixes:

• Add the missing extrapolation of the vtau for MGGA with exponential midpoint rule.
• Fix a missing allocation/deallocation for CAETRS with MGGAs.

Checklist

• 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)