New functions based on the visNetwork package

[rchlumsk]

New functions added to the development (dev branch) version of the package with the visNetwork pacakge! These are added for plotting the model structure (rvn_rvi_process_visnetwork) and subbasin network (rvn_rvh_subbasin_visnetwork_plot).

Take a look at the capabilities of visNetwork package here - https://cran.r-project.org/web/packages/visNetwork/vignettes/Introduction-to-visNetwork.html

Thanks @rarabzad for the contributions, and try out the function examples.