Handle the nested volume in npdet_to_step

**Is your feature request related to a problem? Please describe.**
A (possibly) proper way to retrieve all volumes from the simulation models with `npdet_to_step`

**Describe the solution you'd like**
The user can provide a range of levels, not a single level, as the input parameter to `npdet_to_step` so both the mother and daughter volumes can be converted with a single command. However, I do not know if this would cause additional issues due to overlapping.

**Describe alternatives you've considered**
Convert each level of the nested volumes separately and then assemble them (not preferred).

**Additional context**
An example of nested volumes

Level 2 of RICH endcap (the tank):

Level 3 of RICH endcap (the photosensors and mirrors):