More robust random seeding using a numpy generator

[psomers3]

Summary

Only initialize the seed once on iterator creation. Maybe an option to reset it could be added.
Update This pull request actually needs more work, because it seems like if you want to synchronize two generators (i.e. images and masks) the geomtric tranformations in the ImageDataGenerator can very easily be non deterministic as well.

Related Issues

#351 - not entirely related to this, but in the same direction of truly reproducable results.

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[psomers3]

I saw somewhere this repository is no longer active, so I will close this as it doesn't fully fix any problem.