Add methods to models.iosys for exporting matrices

[pmli]

This PR adds to_* methods to LTIModel and SecondOrderModel, tests for from_* and to_* methods, and output methods to tools.io. The tests should cover most of what is not covered by #1300 in models.iosys. I'm not sure how the CI will react to creating temporary files; I guess we'll see.

[codecov]

Codecov Report

Merging #1309 (49e3a3b) into main (19c2c5b) will increase coverage by 0.34%.
The diff coverage is 91.11%.

Impacted Files	Coverage Δ
src/pymor/tools/io.py	74.07% <73.61%> (+18.83%)	⬆️
src/pymor/models/iosys.py	57.88% <93.10%> (+7.95%)	⬆️
src/pymortests/iosys_save_load.py	100.00% <100.00%> (ø)
src/pymortests/tools.py	96.42% <100.00%> (+0.40%)	⬆️
src/pymortests/vectorarray.py	91.90% <0.00%> (-0.48%)	⬇️
src/pymor/vectorarrays/numpy.py	84.96% <0.00%> (-0.24%)	⬇️
src/pymortests/mpi_run_demo_tests.py	89.74% <0.00%> (+5.12%)	⬆️

[sdrave]

LGTM. However, could you remove the type annotations for now? I don't think it is quite clear at the moment if/when we will have type annotations in pyMOR ..

[pmli]

LGTM. However, could you remove the type annotations for now? I don't think it is quite clear at the moment if/when we will have type annotations in pyMOR ..

I removed the type annotations now.

I also realized that probably save_matrix should be tested as much as load_matrix is. Currently, files in pymortests/data/matrices are used to test load_matrix. How about removing those and testing save_matrix and load_matrix by a round trip as for iosys models?

[renefritze]

I like the current setup as it forces one to consider backwards compat. Maybe we could use the saved (or a new set, since that'll be easier) compare files also for a check if save reproduces them?