Arbor Python objects for decor and cell global properties present opaque interfaces

[halfflat]

arbor.decor or arbor.cell_global_property objects have no methods or attributes that can be used to retrieve any information about them. In principle, a decor object can be queried for its lists of decorations, and similarly cell_global_property should be able to make available its defaults for ions and electrical properties.

Example use case:

p = arbor.neuron_cable_properties()
# How do I discover what the NEURON defaults are at this point?