Create more readable node names in the code

[TimVanMourik]

Currently, a Nipype node creates NodeNAME_XXXX, in the code for a name: A "fsl.bet" node for example creates "NodeName_701a310" in the code.

This should be changes to a more readable name: fsl.bet_001 fsl.bet_002 ...

Such that the user can start hacking code more easily.