Allow muon scattering

[felixhekhorn]

          this isn't always true, right? Compass at least is muon scattering ... yadism and the whole of NNPDF is actually completely lepton-blind, so maybe it is not important, but I believe you can just do an simple `if` similar to the one above

    obs["ProjectileDIS"] = "muon" if "_mup_" in str(src_path.stem) or blub or bla else "electron"

Originally posted by @felixhekhorn in #232 (comment)

          but will yadism work then? 

yadism/src/yadism/coefficient_functions/coupling_constants.py

Lines 346 to 354 in 99d3534

	proj = observables.get("ProjectileDIS", "electron")
	projectile_pids = {
	"electron": 11,
	"positron": -11,
	"neutrino": 12,
	"antineutrino": -12,
	}
	if proj not in projectile_pids:
	raise ValueError(f"Unknown projectile {proj}")

Originally posted by @giacomomagni in #232 (comment)

the answer to the last question is: it won't - let's fix that;

note that "electron" there gets translated to 11 and then later there are a few more of those in the code e.g. here:

yadism/src/yadism/coefficient_functions/kernels.py

Line 142 in 99d3534

if projectile_pid in [-11, 12]:

a relevant function may be is_left_handed_lepton