Create internal functions for adding NHD and ATTAINS geometry to TADA maps

[hillarymarler]

As I started working on updates to TADA_NearbySitesMap and working on how to incorporate ATTAINS assessment units and NHD catchments, I realized that it could be possible to create internal functions to add these features. The internal functions could then be used in both TADA_ViewATTAINS and TADA_NearbySitesMap. Then any needed changes or updates could be made once in the internal functions instead of each mapping function.