Fix debugging for parallelized dfield generation

[rettigl]

The parallelization using multiprocessing recently introduced into the dfield generation prevents the code to run in debug mode (it just freezes or gives errors). This PR replaces multiprocessing by joblib to allow debugging of the code
Also, it removes the additional loop parallelization, as it almost does not improve performance, and complicates the code substantially.

[coveralls]

Pull Request Test Coverage Report for Build 8550711461

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 91.547%

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Totals
Change from base Build 8391068208:	0.2%
Covered Lines:	6011
Relevant Lines:	6566

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💛 - Coveralls

[zain-sohail]

Looks good. There even seems to be an improvement (not sure how reliable) in the benchmark after removal of that extra loop.