StationaryModel with multiple right-hand sides

[pmli]

See https://github.com/pymor/pymor/discussions/1608

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Impacted Files	Coverage Δ
src/pymor/models/basic.py	88.52% <100.00%> (+0.49%)	:arrow_up:
src/pymortests/models/model.py	98.92% <100.00%> (+0.08%)	:arrow_up:
src/pymortests/fixtures/operator.py	98.51% <0.00%> (+0.59%)	:arrow_up:
.../pymor/discretizers/builtin/gui/matplotlib_base.py	57.48% <0.00%> (+0.78%)	:arrow_up:
src/pymortests/strategies.py	88.78% <0.00%> (+1.24%)	:arrow_up:
src/pymortests/vectorarray.py	91.62% <0.00%> (+1.29%)	:arrow_up:
src/pymortests/algorithms/basic.py	98.58% <0.00%> (+1.41%)	:arrow_up:
src/pymordemos/thermalblock_gui.py	79.50% <0.00%> (+1.63%)	:arrow_up:
src/pymortests/demos.py	92.48% <0.00%> (+2.31%)	:arrow_up:
src/pymor/core/defaults.py	79.71% <0.00%> (+2.41%)	:arrow_up:
... and 4 more

[pmli]

Indeed. It would also be nice for discretizers to know how to define multiple right-hand sides, such that it would be easy to define Models for testing.

[sdrave]

@pmli, if this isn't needed urgently, I would propose to wait until we have decided how StationaryModel will look like in the future. Then I would adjust the RB code to also support multiple RHSs.

[pmli]

Sounds good to me.