Checks for inconsistent device types

[jwallwork23]

Closes #269.

This PR adds checks that overloaded operators are only applied to tensors that have the same device type.

It's sufficient to add the checks in the assignment operator and the binary operators as the unary operators will go through the assignment anyway.

The PR also adds checks that tensors have been constructed before they are interrogated. Related fix in the autograd example - the output tensor Q wasn't set up so didn't have a device type yet.

Questions:

• Do we also need to check the device indices are consistent?
• And what about data types?
• Can we add pFUnit unit tests that check these errors are raised as expected?