ttools: make possible null table input values where permitted
ttools: add animation capability to Plot2Task
ttools: allow equality matching of tables in stilts

The ConsumerTask.createProducer method now results in a StarTable instance
which can be compared for equality to other instances obtained in the
same way.  In some cases (plot2) it can give major advantages if two
tables can be identified as equivalent.

It would be even better if the execution Environment could return the
same table instance for separately specified tables.  Maybe in future.
ttools: do plot screen animation on EDT
ttools: multi-thread file output animation in Plot2Task
ttools: only take one RowSequence for Plot2Task animation

This allows it to stream table input from one-time-only tables (e.g. stdin)
to control animation.
ttools: add tloop task