fix bool decoding

in python, any non-empty string is equivalent to True, which
means the saved object 'False' becomes bool('False') == True
allow adding new items to existing data

For example, a new record to an existing bundle or
new attributes to an existing record.

Let's say at time t0, I have a bundle with an entity E1 and an activity A1.
activity A1 has a prov:startTime.

Later, at time t1, I want to add an new entity E2 to that bundle and I want to add prov:endTime to the activity A1.

Since I am not going to write to full provenance data in one shot, I would like to use something like "save_bundle" but that calls for instance:
PDRecord.objects.get_or_create instead of PDRecord.objects.create
and
PDBundle.objects.get_or_create instead of PDBundle.objects.create