(Multi)polygon helper functions

[ajfriend]

Continuation of #1101, please review that first.

• public API functions

  • destroyGeoLoop
  • destroyGeoPolygon
  • destroyGeoMultiPolygon

• internal functions

  • geoPolygonAreaRads2
    • note the logic for computing the area of holes properly
  • geoMultiPolygonAreaRads2
  • createGlobalMultiPolygon
    • We need some multipolygon to return when the user provides, for example, all the cells of a given resolution. Other options could work (like all the res 0 cells), but I think this one is nice because it is fairly simple, all the edge arcs are 90 degrees (our area functions can only handle arcs less than 180), and it has seams along the antimeridian, so other downstream libraries are more likely to be able to process it properly (as opposed to, say, one giant "world polygon")

[coveralls]

coverage: 98.905% (-0.04%) from 98.949%
when pulling fb79132 on ajfriend:cagnoli_area2
into 312fbbc on uber:master.