Avoid conflict when diagnosing `expand_design()` object

[clarabicalho]

• Changes multi-arm estimator column label from "N" to "N_DIM."
• Includes test to avoid conflicting label columns in expanded design diagnosis. More specifically, it checks that no column names in estimate data frame are the same as designer arguments (this would generate errors in DDWizard when it calls diagnose_design() on an object returned by expand_design()).

[coveralls]

Pull Request Test Coverage Report for Build 1365

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0001%) to 99.889%

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Totals
Change from base Build 1359:	0.0001%
Covered Lines:	897
Relevant Lines:	898

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💛 - Coveralls

[jaspercooper]

LGTM, thanks!