ADD: new track message, Entity class and Position class

libs/geographiclib/wrapper/c/README.md

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+# Calling the GeographicLib C++ library from C
+
+The geodesic routines in GeographicLib have been implemented as a native
+C library.  See
+
+  https://geographiclib.sourceforge.io/C/doc/
+
+It is also possible to call the C++ version of GeographicLib directly
+from C and this directory contains a small example, which convert
+heights above the geoid to heights above the ellipsoid.  More
+information on calling C++ from C, see
+
+  https://isocpp.org/wiki/faq/mixing-c-and-cpp
+
+To build and install this interface, do
+```bash
+mkdir BUILD
+cd BUILD
+cmake ..
+make
+```
+This assumes that you have installed GeographicLib somewhere that cmake
+can find it.  If you want just to use the version of GeographicLib that
+you have built in the top-level BUILD directory, include, e.g.,
+```bash
+-D GeographicLib_DIR=../../BUILD
+```
+
+in the invocation of cmake (the directory is relative to the source
+directory, wrapper/C).  To convert 20m above the geoid at 42N 75W to a
+height above the ellipsoid, use
+```bash
+$ echo 42 -75 20 | ./geoidtest
+-10.672
+```
+
+Notes:
+
+* The geoid data (`egm2008-1`) should be installed somewhere that
+  GeographicLib knows about.
+
+* This prescription applies to Linux machines.  Similar steps can be
+  used on Windows and MacOSX machines.
+
+* It is essential that the application be linked with the C++ compiler,
+  so that the C++ runtime library is included.
+
+* In this example, the main program is compiled with the C compiler.  In
+  more complicated situations, it may be necessary to use the C++
+  compiler.  This is necessary to get static initializations of C++
+  classes performed.  (However, GeographicLib doesn't need any static
+  initialization.)