add a "plugin" option for inidissip. Only the interface to the plugin is...

add a "plugin" option for inidissip. Only the interface to the plugin is present in dissip_gcm.F90; a placeholder for a user-provided routine set by an external driven. Default behavior is unchanged as everything is controlled by flag "vert_prof_dissip_type" which is by default "legacy"; set it to "plugin" to use a plugin.