Get first and last coordinate

This adds a couple of small functions `getFirstCoordinate` and `getLastCoordinate` to all simple geometry types. They're roughly equivalent to PostGIS's `ST_StartPoint` and `ST_EndPoint` functions, except they work on all simple geometry types and return an raw array instead of an `ol.geom.Point`. The reason for returning the raw array is that returning an `ol.geom.Point` would currently cause a circular dependency between `ol.geom.SimpleGeometry` and `ol.geom.Point`.