add 'transfer' function that can be called from another script

[momo-the-monster]

also adds TransferParams class that incorporates all known command-line params.

[arthur-fontaine]

@ProGamerGov When are you going to merge? I'd like to use it a function...

[lucasfijen]

Please note that some of the functions, such as setup_multi_device(), setup_gpu() and setup_optimizer still rely on the global params settings in this PR, as they are defined outside of the transfer().

I think that could be best fixed by placing the argparser into the main() function, and adding the params variables in the functions that are outside the transfer(params) function