Functions for spherical calculations

[tschaub]

This adds getLength() and getArea() methods for doing spherical calculations on geometries. By default, the mean radius of the earth is used (from the WGS84 ellipsoid). And by default, geometries are assumed to be in Web Mercator. Both the sphere radius and the projection can be provided as options.

Previously, spherical calculations used by ol.proj.pointResolution() were using the radius for the Clarke 1866 Authalic Sphere. I'm not sure where this came from, but I think using the mean radius with WGS84 ellipsoid values is the right thing to do.

Tacking these functions onto ol.Sphere is awkward, because users never need to construct a sphere. It would be nice to have ol.sphere instead. We can get rid of the ol.Sphere constructor and use ol.sphere in a major release. For now, ol package users can still have nice things:

import sphere from 'ol/sphere';
// ...
sphere.getArea(polygon);

[ahocevar]

Looks great! The measure.html example could be simplified now, using the new functions. But that doesn't need to be part of this pull request.