[minor] Set cwd to None by default

[samwaseda]

Based on the discussion here I bring back the default behaviour.

[samwaseda]

I will merge this as soon as the tests pass

[coveralls]

Pull Request Test Coverage Report for Build 7038764514

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A quick fix for this PR: rebase it; your next report should be accurate.

• 37 of 38 (97.37%) changed or added relevant lines in 2 files are covered.
• 42 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 68.767%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyiron_atomistics/sphinx/output_parser.py	32	33	96.97%

Files with Coverage Reduction	New Missed Lines	%
pyiron_atomistics/atomistics/master/quasi.py	42	25.69%

Totals
Change from base Build 7029482535:	0.1%
Covered Lines:	10806
Relevant Lines:	15714

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💛 - Coveralls