Dynamic properties for ParticleSlice

[mkuron]

Fixes #958.

@RudolfWeeber @fweik @hmenke

Any properties that are not defined for ParticleSlice are automatically drawn from ParticleHandle, including docstrings. To the setter, one can either provide one value that is set on all particles or a list of values where the order corresponds to that of the slice members.

This turned out to be surprisingly difficult:

• One cannot use setattr to add stuff to Cython classes, so I ended up wrapping the Cython class into a Python class that has all the autogenerated properties.
• The wrapping cannot be done in the loop I added to the end of particle_data.pyx. I had to introduce a class that holds the attribute name as a member. If I generated the functions directly in the loop (i.e. capturing the loop variable), the attribute name a setter/getter was operating on would vary in a non-deterministic fashion (seems like some kind of reference counting bug in Python itself...).

Further improvements made along the way:

• Fix ParticleSlice.__str__, which previously didn't return anything as int is not the same as numpy.int64.
• Improve ParticleSlice.__str__ and ParticleList.__str__ so that the representations look like Numpy arrays or Python lists. Previously there were newlines and the name was missing.
• Improve utils.check_type_or_throw_except so that it also allows numpy.int64 where int is requested.

PR Checklist

• Tests?
  • Interface
  • Core

[KaiSzuttor]

Maybe add some more in-code documentation for future developers.

[mkuron]

@KaiSzuttor, I'm currently refactoring _InjectParticleProperty to make it easier to understand. That should address your documentation request.

[hmenke]

No need for this awkward comma removal

pl = ParticleList()
return "ParticleSlice([" + ", ".join(str(pl[i]) for i in self.id_selection if pl.exists(i)) + "])"

[mkuron]

It's nicer, and it's more in line with how Python and Numpy print their own data types. Also, if I remove that space at the end, why not remove the trailing comma along with it.

[hmenke]

No need for this awkward comma removal

return "ParticleList([" + ", ".join(str(self[i]) for i in range(max_seen_particle + 1) if self.exists(i)) + "])"

[mkuron]

I was able to get rid of the injector class by switching to functools.partial. It seems like the following two are different here, even though they look like they do the same:

functools.partial(geta, attribute=attribute_name)
lambda s: geta(s, attribute_name)

Explicitly capturing the loop variable like this

def geta(particle_slice, attribute=attribute_name):

didn't help either, even though that is the standard solution for this problem in pure Python. I really don't understand what is going on here, might be a Cython bug.

Anyway, once Travis passes, I think this can be merged.

[mkuron]

Small change to the getter so it writes straight to a numpy array instead of converting a list into one at the end. Also extend unit test so it covers all code paths.