feat: Allow duplicates to be dealt with in parse_survey

[jamieRowen]

Looks to address #74 by allowing parse_survey to proceed even if duplicate data are found. The existing solution errors as a result of {assertthat} and has no escape hatch.

This PR adds a parameter to parse_survey, fix_duplicates, which defaults to the same error behaviour as is currently present with an expanded message. But also allows the user to specify that duplicates are kept or dropped as part of the processing of survey responses. In each case if duplicates are found a warning will be presented.

• test: Add tests for functions dealing with duplicates
• default is to retain the error behaviour before

[jamieRowen]

@mattroumaya Changes made as requested