[minor] Don't have a default Executor

[liamhuber]

Make the user choose one specifically.

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👈 Launch a binder notebook on branch pyiron/pyiron_workflow/no_default_executor

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Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (0e3a558)	3472	3208	92.40%
Head commit (3f1d965)	3470 (-2)	3206 (-2)	92.39% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#390)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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