Surface Quantities: Surface Flux

[jhdark]

Proposed changes

With this change users will now be able to export the surface flux at a given surface subdomain at each time step using:

my_species = F.Species("A")
my_subdomain = F.SurfaceSubdomain1D(id=1, x=1)

my_export = F.SurfaceFlux(
        filename="surface_flux.csv",
        field=my_species,
        surface_subdomain=my_subdomain,
    )

With this implementation, the filename is optional, however, the export data will still be accessible after the simulation. Additionally, the time of the simulation is also saved at each iteration:

simulation_times = my_export .t
flux_values = my_export .data

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...