Cleaned up `CMakeLists.txt` files in subdirectories so that the minimum reqiured
version is set only at the beginning of the top-level `CMakeLists.txt` file

Fixup makefiles so that these examples compile either as single directories or as part of a build.  Do not require special versions of GeoModel:  I will propose that these examples be distributed not as useful compiled executables, but rather as source code, for people to learn from.  Additionally, I believe that these should be carefully scrutinized and annotated with abundant comments for pedagogical purposes, and probably also referenced in the documentation.