Consider making all immutable ZTensors a subclass

[crutcher]

When using ZTensors as part of immutable datastructures, it is useful to have solid requirements that they are treated as immutable objects.

Currently, this is done by construction enforcement (t.asImmutable()); or by immutable wrapper classes (ZPoint, ZMatrix).

conceivably, there could be two subclasses of ZTensor, FixedZTensor and MutableZTensor.

Properly handled, algorithms which didn't need to know/care could be written for the base type (ZTensor).

Achieving this would require some recursive template dancing, and private static dispatch constructors which switched on mutable.