fix: various RNTuple fixes

[ariostas]

Very simple RNTuple files can be read, but currently there are issues reading more complex files. In this PR, I'll be working towards fixing as many issues as I can find. I'll update this description with more information once it's ready for review.

[jpivarski]

Very nice! I've scanned through everything and it's clear and self-explanatory. It looks like some dtypes changed, or maybe they're only getting tested now.

Please merge this to confirm that you're done with the PR. Thanks!

[jpivarski]

FYI, one of the tests in tests/test_1191_rntuple_fixes.py is pretty slow. My test-runner seemed to be stuck indefinitely, so I reran it with --durations=0. It was a lot faster the second time, so it must have been stuck on something the first time, but still, 5 seconds is a bit long.

==================================== slowest durations =====================================
5.58s call     tests/test_1191_rntuple_fixes.py::test_multiple_page_lists
0.21s call     tests/test_1191_rntuple_fixes.py::test_schema_extension
0.11s call     tests/test_1191_rntuple_fixes.py::test_skip_recursively_empty_structs
0.03s call     tests/test_1191_rntuple_fixes.py::test_empty_page_list
0.02s call     tests/test_1191_rntuple_fixes.py::test_rntuple_cardinality
0.00s call     tests/test_1191_rntuple_fixes.py::test_split_encoding
0.00s setup    tests/test_1191_rntuple_fixes.py::test_schema_extension
0.00s teardown tests/test_1191_rntuple_fixes.py::test_multiple_page_lists
0.00s teardown tests/test_1191_rntuple_fixes.py::test_skip_recursively_empty_structs
0.00s teardown tests/test_1191_rntuple_fixes.py::test_schema_extension
0.00s setup    tests/test_1191_rntuple_fixes.py::test_split_encoding
0.00s setup    tests/test_1191_rntuple_fixes.py::test_empty_page_list
0.00s teardown tests/test_1191_rntuple_fixes.py::test_split_encoding
0.00s setup    tests/test_1191_rntuple_fixes.py::test_rntuple_cardinality
0.00s teardown tests/test_1191_rntuple_fixes.py::test_rntuple_cardinality
0.00s teardown tests/test_1191_rntuple_fixes.py::test_empty_page_list
0.00s setup    tests/test_1191_rntuple_fixes.py::test_skip_recursively_empty_structs
0.00s setup    tests/test_1191_rntuple_fixes.py::test_multiple_page_lists

Maybe it was stuck downloading http://root.cern/files/tutorials/ntpl004_dimuon_v1rc2.root.

We have a network pytest mark to label tests that have to access the network, so that we can say

pytest -m "not network" tests

to exclude them on a quick-test run. That would be good to add, in the future.

[ariostas]

Ah okay, sorry about that. I'll try to find/generate a replacement file. Thanks @jpivarski