Switch particle IDs to a more widely used definition

[joheinze]

The code currently uses the NEUCOSMA ID convention. This means particels are defined by

• 0: photon
• 2-4: pions
• 5-10: muons (seperate ids for different helicities)
• 11-16: neutrinos
• 20,21: electron, positron
• 50,51: kaons
• A x 100 + Z: nuclei

This should eventually be moved to a more conventional sheme, e.g. the PDG Monte Carlo numbering scheme (also CRPropa uses).

Unfortunately the indexing sheme is hard coded in several places in the cross_sections and decays modules. E.g. by code sections like this:

PriNCe/prince/cross_sections/base.py

Lines 193 to 197 in aed63e5

	for mo, da in self.incl_idcs:
	if da >= 100 and get_AZN(da)[0] > get_AZN(mo)[0]:
	raise Exception(
	'Daughter {0} heavier than mother {1}. Physics??'.format(
	da, mo))

Which should rather make used of the PriNCeSpecies class like this:

        for mo, da in self.incl_idcs:
            if da.is_nucleus and da.A > mo.A:
                raise Exception(
                    'Daughter {0} heavier than mother {1}. Physics??'.format(
                        da, mo))

Several similar checks occur throughout the code.
However this affects the cross section class structure, since self.incl_idcs is a dictionary of tuples (and PriNCeSpecies cannot be used as a dictionary key).

The same goes for the used of get_AZN throughout the code, which was not fully adressed in issue #2. This issue is also related to #4 and #6, since the data files contain the NEUCOSMA IDs explicitly

[joheinze]

Since this will be somewhat more work, I do not think this issue has to be adressed before publishing the code. So I separated this from issue #2.