Fix BasicTumbleStatus to initialize drag to non-zero.

computeTumbleDrag() is never called without this fix.

The one-arg constructor is the only constructor ever used, to init this
status based on the previous status, copying the tumble drag if it can.

The first BasicTumbleStatus is constructed from an RK4 Status, so it's
drag is not copied, but neither is the drag computed. From then on the 0
is copied from one BasicTumbleStatus to the next.
The changes to the tumble transition discussed on or-dev
Merge pull request #182 from enderw88/master

Prevent NPE when an uninitialized table cell is rendered.
Merge pull request #181 from bkuker/tumble

Changes to Tumble Transition & Tumble Bug Fix