Introduce an algorithm class

Description

Several changes related to the multisystem propagators:

• some cleaning
• add an algorithm class that implements the concept of algorithms as lists of algorithmic operations.
• use the new classes for the propagators
• move the description of the algorithmic operations specific to a given propagator to the corresponding module

The long term idea is that each system should have an algorithm that is executed during the calculation. This might be something different than a TD propagator, like an energy minimization. Before we get there, more changes are required to the framework, but this will be done over several merge requests.

Contributes to #376 (closed)

News snippet

Introduce an algorithm class.

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.

changed milestone to %11.0

added Multisystem label

added 2 commits

• a6bf7051 - Move algorithmic operations that are specific to certain propagators to the corresponding module.
• ec5f61d8 - Move the existing propagators to a new folder named algorithms.

Compare with previous version

Codecov Report

Merging #1026 into develop will increase coverage by 0.02%. The diff coverage is 69.01%.

@@             Coverage Diff             @@
##           develop    #1026      +/-   ##
===========================================
+ Coverage    68.36%   68.38%   +0.02%     
===========================================
  Files          512      513       +1     
  Lines        97906    97872      -34     
===========================================
- Hits         66930    66927       -3     
+ Misses       30976    30945      -31     

Impacted Files	Coverage Δ
src/classical/charged_particle.F90	92.30% <ø> (-0.55%)
src/electrons/electrons.F90	65.83% <ø> (+1.59%)
src/hamiltonian/gauge_field.F90	85.05% <ø> (-0.08%)
src/multisystem/system.F90	74.13% <0.00%> (-0.20%)
src/td/propagator_elec.F90	86.70% <25.00%> (ø)
src/multisystem/propagator_beeman.F90	57.14% <43.75%> (ø)
src/multisystem/propagator_exp_mid.F90	61.29% <52.63%> (ø)
src/classical/classical_particle.F90	88.65% <100.00%> (-0.19%)
src/maxwell/system_mxll.F90	84.16% <100.00%> (-0.26%)
src/multisystem/algorithm.F90	100.00% <100.00%> (ø)
... and 5 more

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assigned to @nicolastd

@micael.oliveira Apart from minor comments, this looks ready to be merge. Once you address them, I will merge it.

added 4 commits

• c2d3eda4 - Add new algorithm and algorithmic_step classes.
• a5c6acfa - Using the new algorithm class in the propagator.
• 7f32dc35 - Move algorithmic operations that are specific to certain propagators to the corresponding module.
• b8592731 - Improve description of several algorithmic operations.

Compare with previous version

added 6 commits

• 0e711fbe - Deleted the store_current_status defered method from the system class. Either...
• a13acee8 - Delete ununsed SYNC propagator operation.
• 676cc6c7 - Add new algorithm and algorithmic_step classes.
• 5e2f751b - Using the new algorithm class in the propagator.
• e49abfa1 - Move algorithmic operations that are specific to certain propagators to the corresponding module.
• 7280707f - Improve description of several algorithmic operations.

Compare with previous version

changed the description

assigned to @heiko.appel

added 4 commits

• ea46e8b2 - Add new algorithm and algorithmic_step classes.
• 1915977c - Using the new algorithm class in the propagator.
• 541437f1 - Move algorithmic operations that are specific to certain propagators to the corresponding module.
• 79672c21 - Improve description of several algorithmic operations.

Compare with previous version

resolved all threads