Add function to merge unspecified number of data frames into final data frame
The current best-practice approach for the data processing pipelines is to have multiple processing functions create separate data frames (e.g., one data frame for session detail variables, another for time-line follow-back measures, etc.). This approach integrates nicely with the visualization tools in 'targets'. However, the final step requires combining multiple data frames into the single output data frame. A standardized function for this final step should be defined to ensure consistency across projects.