AST-1572: speed up h5parm reading

Find the rationale of the solution in the ticket https://jira.skatelescope.org/browse/AST-1572 and also in the comments in file https://git.astron.nl/RD/schaapcommon/-/blob/ast-1572-speed-up-h5parm-reading/src/h5parm/h5cache.cc

requested review from @mnijhuis-tos and @dijkema

assigned to @nicolas-slusarenko

Very nice improvement! I only found some style issues and a few simplications, but the overall design is perfect.

requested changes

resolved all threads