Add new propagation worker and rewrite propagators to use it

Description

Create a generic electronic worker which 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.

News snippet

Reorganization of the code of the propagator.

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.