Add Unit checking

[ewanwm]

There are a number of libraries to do compile-time and run-time dimensional analysis. This would be a very useful feature to add to tensor operations. I think it would be good to have both: compile time checking for c++ to have no impact on run time speed, but run-time checking will need to be enabled when using python. I think best way to do this would be adding some CHECK_UNITS macro or something like this and implementation can be swapped out depending on cmake options.