[ENH] Improvements to DWI preprocessing pipelines

[NicolasGensollen]

Remaining bits of #989 which propose various improvements to the DWI preprocessing pipelines:

• more explicit names
• improved documentation
• simplification of connection specifications when possible
• avoid writing files in input BIDS folder
• define a few nipype interfaces as wrappers around properly implemented functions with type hints and without self imports when using helper functions