Merge OpenStreetMap peaks not in BIH data and improve display of distant hills

Add instructions for merging OSM with BIH hill data, preferring BIH hill data where there is an overlap based on distance between peaks and BIH hill height + drop. New hills database included brings back most of the local hill data from OSM.

Re #9, increase number of distant hill labels by displaying remaining hills at the minimum label font size. Increase minimum alpha to assist with readability.

Fix #15 by tweaking label display logic and a bit of refactoring. Not perfect but hopefully better!