Select RTE solver via option

[riclarsson]

This simplifies how the radiative transfer equation is chosen.

You now have to set

ws.rte_option = "optional value"

to select equation. This new WSV is of type TransmittanceOption, and chooses how the new TransmittanceMatrix struct is initialized. By default, this still falls back to "constant", which is the old

I1 = J + T (I0 - J)

The other two options uses linear-in-tau or linear-propagation solvers.

I will not make the switch from this RTE to the linear-in-tau as part of this branch.

[Copilot]

Pull request overview

This PR simplifies the radiative transfer equation (RTE) solver selection by introducing a new rte_option workspace variable of type TransmittanceOption. Instead of using different agenda methods, users now set ws.rte_option to "constant", "lintau", or "linprop" to select the RTE solver.

Changes:

• Introduced new rte_option WSV with three options for selecting RTE solvers
• Refactored transmission and source calculations into unified TransmittanceMatrix and SourceVector structs
• Removed deprecated agenda-based methods (spectral_radClearskyLinearInTauEmission, etc.)

Reviewed changes

Copilot reviewed 51 out of 51 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/core/**/*.py	Updated test files to use new ws.rte_option instead of agenda methods
src/workspace_variables.cpp	Added rte_option WSV, updated variable descriptions for new types
src/workspace_groups.cpp	Added TransmittanceMatrix, SourceVector, StokvecTensor3, TransmittanceOption types
src/workspace_methods.cpp	Removed old methods, updated method signatures to use new types
src/workspace_meta_methods.cpp	Removed linear-in-tau specific meta methods
src/m_tramat.cc	New file implementing spectral_tramat_pathFromPath with unified interface
src/m_srcvec.cc	New file implementing spectral_rad_srcvec_pathFromPropmat with unified interface
src/m_spectral_radiance.cc	Simplified emission calculations using new unified types
src/core/rtepack/rtepack_transmission.*	Implemented TransmittanceMatrix struct with option-based initialization
src/core/rtepack/rtepack_source.*	Implemented SourceVector struct with unified interface
src/core/rtepack/rtepack_rtestep.*	Added new RTE step functions supporting all three options
src/core/options/arts_options.cc	Added TransmittanceOption enumeration with documentation
examples/**/*.py	Updated examples to use new rte_option syntax

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