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)
- Draft implementation (for now only implemented for +proj=isea +R=6371007.18091875 with no other parameters)