update fread in reading functions

[rchlumsk]

Need to update packages to use fread in:

• rvn_hyd_read
• rvn_forcings_read
• rvn_res_read
• rvn_wshed_read
• rvn_exhaustive_mb_read
• rvn_custom_read
• any others I may have missed

Check the importFrom in each package header and ensure fread is being sourced properly, may be an issue with how it is currently done via:
@importFrom data.table fread