Support manual filters in RTDCBase hierarchy children

[paulmueller]

https://github.com/ZELLMECHANIK-DRESDEN/dclab/blob/efec8a7287a78be5a7baba1db8ed780295b3d333/dclab/rtdc_dataset/fmt_hierarchy.py#L103-L105

Currently, after each call of apply_filter for a hierarchy child, its filtering data is reset, which makes it impossible to use manual filters. The solution is to override _init_filters in RTDC_Hierarchy and determine which events were selected manually. This has to be done on the level of the root hierarchy parent (because a hierarchy parent can also be a hierarchy child). Some logic is required to map the index of the hierarchy child to the index of the root hierarchy parent and back.