More pythonic membership on mechanism_catalogue. (#1306)

Introduce two minor changes to the Python API
to handle mechanism_catalogues idiomatically.

Instead of

import arbor as A

cat = A.default_catalogue()

if cat.has('hh'):
  print("Found HH.")

for mech in cat.keys():
   print("*", mech)

we can now write

import arbor as A

cat = A.default_catalogue()

if 'hh' in cat:
  print("Found HH.")

for mech in cat:
   print("*", mech)

which is closer to the expectations of Python users.