New function params from reviewer comments, documentation updates

[dklinges9]

Set of commits stemming from addressing reviewer comments and improving modularity while I was at it.

Highlights:

• users can specify whether they wish to combine multiple downloaded netCDF files (one for each year) into one file
• users can specify if they want the hourly precip values rather than the (default) daily sum
• checks whether user will overwrite a file with a new query and will stop them unless they specify overwrite = TRUE
• updating the vignette to reflect these changes, and some adjusts and improves of function documentation