Improvements on detection of the GEOS library

* Added a FoundGEOS CMake module
* Added a CMake macro for setting specific GEOS version variables
* Updated CMake build configuration with FindGEOS usage
* Fixed Factory class definition to avoid cyclic dependencies 
  between OpenFLUID libraries

(reference #32)
Improvement of the FindGEOS CMake module

* Added GEOX_xxx variables to cache to avoid multiple run of the module
  during the build configuration

(references #32)