[DRAFT] Test optimal kerchunk

[valeriupredoi]

Description

This is a sandbox testing a slightly modded Kercunk that kerchunks inly a desired Dataset - the good folk at Kerchunk are already investigating means of implementing such an option in Kerchunk in fsspec/kerchunk#424 - and some timing test results can be seen in fsspec/kerchunk#424 (comment) (after one sets default_cache_type to first as Martin recommended)

Label: needs new Kerchunk

Update as of February 29

With the Kerchunk PR now merged fsspec/kerchunk#424 and the issues related to the newer Kerchunk functionality now ironed out, this is ready for a merge when kerchunk release the new version that contains that PR. Test results are very promising:

Test time / time it spends before going to remote Reductionist
--------------------------------------------------------------
4.87        (0.71)
5.05        (1.06)
7.02        (3.03)
4.84        (0.81)
4.72        (0.74)
4.77        (0.73)
4.75        (0.78)

So all the time spent in PyActiveStorage is sub-1s! This is for the tests/test_compression_remote_reductionist.py::test_compression_and_filters_cmip6_forced_s3_from_local_bigger_file_v1 test.

[valeriupredoi]

OK new Kerchunk=0.2.4 works very well, and I also fixed the dreaded SegFault that was plaguing us until today 🥳