Dsl/temporal operators

[HenningScheufler]

Motivation

adds implicit temporal operator

included features in NeoFOAM:

• replaced Operator by Spatial and TemporalOperator

Advantages:

• one interface for spatialOperators and one for temporalOperators (Interface segregation principle)
• No global timestep object is required. t and deltaT can easily be injected and the interface can be adapted if necessary
• Operators are now either explicit or implicit. (Spatial and Temporal is included in the type)

Cons:

• a bit more complex

included features in FoamAdapter:

• test_implicitOperators
• SourceTerms: explicit and implicit
• DivOperators: explicit and implicit
• TemporalOperators: explicit and implicit

ToDo:

• improve code:
• introduce new enum so we dont rely on SpatialOperator::Type
• remove NeoFOAM/dsl/ddt.hpp?

[github-actions]

Deployed test documentation to https://exasim-project.com/NeoFOAM/Build_PR_259

[HenningScheufler]

@gregorweiss @greole @bevanwsjones @MarcelKoch

I would propose to not add any more features to this PR and focus on improving code quality and merge it as quickly as possible in stack/implicitOperator.

Currently, multiple PR adapt the DSL and introduce new features. If we quickly merge this PR, we can minimize the impact on the other PRs.

The next Milestone would be to implement scalarAdvection with implicitOperators:

With ideally required the following features:

• runtime-selectable solvers
• GPU solver support
• handling of implicit boundaries

[HenningScheufler]

auto is a bit miss leading here

[greole]

some quick comments

[greole]

remove usage of namespace NeoFOAM:: inside the NeoFOAM namespace.

[greole]

can we use x or solution instead of psi?

[greole]

what is diagIndex? Please add a docstring

[greole]

using op would be more consistent, cf with loop in L.44

Suggested change

	for (auto& oper : spatialOperators_)
	for (auto& op : spatialOperators_)

[greole]

So most of the implementation is gone from here: can this file be removed all together otherwise maybe add some docstring why the Operator class is still needed and what the purpose is.