implemented simple connection slicing
Moved Neurons and ObjSlice to objects.py

- Also patched some odd issues with deepcopying np.sin on Python3
made Node dimensions mandatory

this was required for EnsembleArray slicing

also added some more tests
Simplified Connection checks
Connection.transform is now unpadded

The padded version is available as Connection.transform_full
Quick patch for test_context.test_with

- Not a permanent thing!
Ensure builder does not modify transform_full

By making it have a handle to a copy.

Also updated the Connection docstring.