National Geographic is looking to help factual fans binge box-set documentaries as part of the launch of a video-on-demand service across Europe. The brand is doubling down on digital with National Geographic+, which will launch in Denmark, Greece and South Africa in 2018. National Geographic+ will target younger, millennial viewers and feature a curated collection of content across topics including space, Egypt, world cultures, deep-sea exploration and the human body. It…