Problem: the JGF reader requires each vertex's 'properties' field
to be either null or a JSON object, but doesn't ever check the type.
This leads to silent failures when the type read in is wrong.

Add a check for the type of the properties field if it is not null.

Problem: the Python JGF writer outputs properties as a list by
default, but the reader expects a mapping.

Change the default to a dictionary.

Problem: there are no tests that the JGF writer handles
properties correctly.

Add them.

Problem: t/t8001-util-ion-R.t expects JGF properties to be formatted
as a JSON array, but they are actually given by a mapping.

Fix the expected type.