Sptensor: add support for constructing from data without copying

Follow up to 
- #145 

Follow changes from #182 for `ktensor` as a guide.

Constructors:
- Update: `__init__`: (replaces `from_data`, adds copy parameter)
- Update: `from_function`
- Update: `from_aggregator`
- Remove: `from_tensor_type`
- Remove: `from_data`

Add methods:
- `copy`