Handle the nested volume in npdet_to_step

[Chao1009]

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):