Support user-defined filters

[HDembinski]

One of the main advantages of using Python in the CLI is that we can let the user write some Python code to filter the raw event produced by impy. We add a command line argument to the CLI, which points to user-defined a Python file which contains the filter code, which applies transformations on the EventData object.

impy --filter=my_filter.py ...

my_filter.py is a normal Python file. We expect that a function filter(event) is defined. It should accept the original event and return a modified event. Example:

# keep only long-lived particles
from impy.constants import long_lived

def filter(event):
     apid = np.abs(event.pid)
     ma = False
     for llpid in long_lived:
           ma |= apid == llpid
     return event[ma]  # or return None to discard the event completely

We can ship a couple of standard filters with the library, like we already do (filters like EventData.final_charged() etc. ... already exist), but it would be very useful to expose this generic functionality via the CLI.