- Integrating code ported from Java to eC, then eC to C++ implementing the inverse ISEA projection
- Originally from Franz-Benjamin Mocnik's ISEA implementation found at
   https://github.com/mocnik-science/geogrid/blob/master/src/main/java/org/giscience/utils/geogrid/projections/ISEAProjection.java
   (MIT License)
- NOTE: The inverse only supports the default planar options

- Eliminating several redundant trigonometric function calls
- Single shared definition of dodecahedron vertices
  (centers of icosahedral triangular faces) for
  forward and inverse
- Shared data structure for forward and inverse
- Consistent use of 0-base index for triangular faces

- NOTE: There may have been an undocumented +rescale= parameter which
will no longer work

- Avoid calling isea_rotate() in isea_tri_plane() to rotate
  coordinates in bottom triangles by 180 degrees.