Add support for multiple dynamic dimensions and to_pandas conversion for dynamic arrays
Add support for multiple dynamic dimensions (e.g. `(None, None, 3)` for arbitrary sized images) and `to_pandas()` conversion for dynamic arrays.

TODOs:
* [ ] Cleaner code
* [ ] Formatting issues (if NumPy doesn't allow broadcasting even though dtype is np.object)
* [ ] Fix some issues with zero-dim tensors 
* [ ] Tests
