Add examples that use transform= argument to the tutorials

[selitvin]

trafficstars

We should add more documentation of this feature. We can stress that together with ProcessPool, this enables tensorflow users to do heavy preprocessing by writing Python code (tensorflow's native py_func is limiting as it takes the GIL.transform= feature of Petastorm can help a lot in this case)

[jsgoller1]

Claiming this issue (assuming it's still open/valid).