Improve validate() performance for large arrays of structures

[maarten-ic]

Prevent an expensive _path calculation when it is not required.

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Speeds up ids.validate() by a factor 5 on SDCC for the example below:

import timeit
import imas
from imas.ids_defs import CLOSEST_INTERP

uri = "imas:hdf5?path=/home/ITER/tribolp/public/imasdb/interp/105027/from_predictMagnetics_5kHz"
with imas.DBEntry(uri, "r") as entry:
    mag = entry.get_slice("magnetics", 0, CLOSEST_INTERP)

print(timeit.repeat("mag.validate()", number=10, globals=globals()))

The changes in this PR reduce the time for a single validate() from 370 ms to 66 ms on an SDCC login node with toolchain 2023b.

[olivhoenen]

Nice improvement, thanks!