Change worker_list file to node_list

[shuds13]

Name change of default node_list file in user run directory.

For anyone specifying a libEnsemble nodelist through a file, the default name of this file will change to node_list (previously worker_list was used, intended as the nodes available to workers).

If present in the run directory, it specifies the whole node list available to libEnsemble, as an alternative/override to auto-detection.

[coveralls]

Pull Request Test Coverage Report for Build 3548

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 96.334%

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Totals
Change from base Build 3543:	0.03%
Covered Lines:	4458
Relevant Lines:	4587

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3546

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.302%

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Totals
Change from base Build 3543:	0.0%
Covered Lines:	4457
Relevant Lines:	4587

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💛 - Coveralls