v2.1.7 release

RavenR version 2.1.7 now on CRAN and released here.

Minor updates and bug fixes, including:

• added new functions including rvn_rvp_fill_template for filling in rvp template files, rvn_met_recordplot to show station record lenghts;
• added function for plotting subbasin network with interactive visNetwork library
• updated rvn_run with additional options, including adding rvi commands to the file before execution;
• added new model templates from Raven Manual v3.5 to rvn_rvi_write_template;
• enhancements to rvn_rvh_read for splitting subbasin and HRU information in separate files, other minor improvements; and
• bug fix in rvn_rvt_write for writing initial start date with correct month code;