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.

src/maxwell/system_mxll.F90

@@ -393,6 +393,10 @@ contains
     select case (operation%id)
     case (SKIP)
       ! Do nothing
+
+    case (STORE_CURRENT_STATUS)
+      ! For the moment we do nothing
+
     case (EXPMID_START)
       SAFE_ALLOCATE(this%rs_current_density_ext_t1(1:this%gr%mesh%np_part,1:this%st%dim))
       SAFE_ALLOCATE(this%rs_current_density_ext_t2(1:this%gr%mesh%np_part,1:this%st%dim))