Small refactoring in realm.py

realm.py already had  ``` ***_by_type ``` functions.

Replaced ```get_all_subs_[type]``` functions by ```get_all_subs_satellites_by_type(sat_type)```
This merges 2 functions into 1.

Replaced ``` fill_potential_[type] ``` functions by ``` fill_potential_satellites_by_type ```
This merges 4 functions into 1.