Support general cores in TTENSOR

[ntjohnson1]

This added ttensor support for dense cores. MATLAB supports any core that follows an interface. We can probably use protocols and mypy to sanity check this. Alternatively may consider creating a base class for some shared pyttb tensor functionality.

[DeepBlockDeepak]

Leaving note here to highlight issue #147 's Core Type Conversion note, and Nick's response to it. The Ktensor.ipynb will need to be updated accordingly, once this this core issue is resolved.

[ntjohnson1]

MATLAB states generic cores are supported but special cases tensors and sptensor. For the scope of this ticket only adding support to sptensor.