`simple_data_loader` isn't very simple

[ts4051]

simple_data_loader is becoming increasingly bloated with bespoke behaviour and is no longer really simple.

We should make a dedicated data loader for our IceCube PISA case containing all this functionality, and then make simple_data_loader back into a basic "load a HDF5 file as it is" stage which will be more useful for general cases (e.g. keeping PISA a tool with applicability beyond IceCube).

I am happy to do this when I revisit the events class soon (to better integrated with containers, etc).