Refactor HDF5 and preserve tree structure (#7743)

* recursive

* refactor

* simplify vlen

* rename

* rm empty __post_init__

* always cast

* don't always cast complex parts to float64

* format

* rm hasattr metadata

* update tests

* update comment

* rename

* fix test

* fix copypaste

* check complex splits data into real/imag

* fix prefix, empty groups

* handle chunkedarray gracefully

* fix empty file test

* format

Author: klamike