Refactor data_transform to allow datum, cv:Mat and Blob transformation

Move data_mean into the data_transform class to facilitate data transformation. 
This way Data layers don't need to hold it and pass it all the time. 

Yeah I used for benchmarking, I will remove them and create a new PR for benchmarking. I think it will be nice to have a new FLAG TIMING with different levels that could be used to enable timings.