Utility function PTC::Reader for getting the stored frame range.

The way this works is currently not ideal, it takes the start/end times
used by Alembic and then multiplies by the scene fps to convert into
frames. It might be better to use the sample numbers directly, but
we cannot associate these directly to frames so far.
Store the scene pointer directly in reader/writer base class. This is
required for a variety of uses, mostly related to fps calculation atm.