task runtime: option to copy input files and mount them read/write

[mlin]

cf. #210

This implements the option to run_local_task but doesn't yet expose it through run_local_workflow or the CLI, to avoid creating merge conflicts with #229

[coveralls]

Pull Request Test Coverage Report for Build 1095

• 45 of 45 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 96.881%

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Totals
Change from base Build 1084:	0.009%
Covered Lines:	4317
Relevant Lines:	4456

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💛 - Coveralls