Export all results with "cg_" prefix

[anthonysena]

The cohort generator results csv files should have, by default, the "cg_" prefix as is specified in the resultsDataModelSpecification.csv (ref
#154). Currently, the csv output does not include the "cg_" prefix to preserve the current, expected output. For the next major release, we should change this behavior to always include the "cg_" prefix which will be a breaking changes for others that use the CohortGenerator package.

[anthonysena]

Additionally we should remove the parameters snakeCaseToCamelCase and fileNamesInSnakeCase since these were to maintain compatibility with CohortDiagnostics. Instead we should simply follow the HADES standard https://ohdsi.github.io/Hades/codeStyle.html#Interfacing_between_R_and_SQL.