ISEA Inverse projection (#3047)

- 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

docs/source/operations/projections/isea.rst

@@ -4,18 +4,24 @@
 Icosahedral Snyder Equal Area
 ********************************************************************************
 
-Snyder's Icosahedral Equal Area map projections on polyhedral globes for the
-dodecahedron and truncated icosahedron offer relatively low scale and
-angular distortion. The equations involved are relatively straight-forward,
-and for certain instructional tools and databases, the projections are useful
-for world maps. The interruptions remain a disadvantage, as with any low-error
-projection system applied to the entire globe :cite:`Snyder1992`.
-
+Snyder's Icosahedral Equal Area map projection on an icosahedron polyhedral globe
+offers relatively low scale and angular distortion. The equations involved are
+relatively straight-forward. The interruptions remain a disadvantage, as with
+any low-error projection system applied to the entire globe :cite:`Snyder1992`.
+This projection is used as a basis for defining discrete global grid hierarchies.
+
+.. note::
+   As the projection is only defined on a sphere, it should only be used
+   with a spherical ellipsoid e.g., ``+R=6371007.18091875`` for a sphere with the
+   authalic radius of the WGS84 ellipsoid. For mapping coordinates on the WGS84
+   ellipsoid to the authalic sphere, the input latitude should be converted
+   from geodetic latitude to authalic latitude. A future version may
+   automatically perform this conversion when using a non-spherical ellipsoid.
 
 +---------------------+----------------------------------------------------------+
 | **Classification**  | Polyhedral, equal area                                   |
 +---------------------+----------------------------------------------------------+
-| **Available forms** | Forward, spherical                                       |
+| **Available forms** | Forward and inverse, spherical                           |
 +---------------------+----------------------------------------------------------+
 | **Defined area**    | Global                                                   |
 +---------------------+----------------------------------------------------------+
@@ -44,27 +50,35 @@ Parameters
 .. option:: +orient=<string>
 
     Can be set to either ``isea`` or ``pole``.  See Snyder's Figure 12 for pole orientation :cite:`Snyder1992`.
-    
+
     *Defaults to isea*
 
 .. option:: +azi=<value>
 
     Azimuth.
 
+    Not supported by the inverse.
+
     *Defaults to 0.0*
 
 .. option:: +aperture=<value>
 
+    Not supported by the inverse.
+
     *Defaults to 3.0*
 
 .. option:: +resolution=<value>
 
+    Not supported by the inverse.
+
     *Defaults to 4.0*
 
 .. option:: +mode=<string>
 
     Can be either ``plane``, ``di``, ``dd`` or ``hex``.
-
+
+    Only ``plane`` supported by the inverse.
+
     *Defaults to plane*
 
 .. include:: ../options/lon_0.rst