Be friendly to super project

This MR aims to address #25 (closed)

Current list of changes (last update 8 March 2020):

1. googletest removed from source tree. tests rely on system-wide installation of gtests
2. headers moved to ./include
3. cmake 2geom targets are exported under 2Geom:: namespace and installed
4. minor CMakeLists.txt simplifications (remove redundant repeated dependencies)
5. test dependent project usage of lib2geom a) as sub-project b) as installed library (via find_package)

Closes #25 (closed)