Fix minor issues in CMake setup

There are a few minor issues in the current CMake setup

1. Output is not always consistent (e.g. in FindSpdlog.cmake)
2. Locally installed versions of Eigen3/spdlog are preferred over the submodules (it should be the other way around)
3. CMake setup fails on machines without OpenMP
4. Minor spelling issues in output messages
5. Coverage should only be enabled if testing is active
6. Add spdlog as dependency of installed elsa
7. When using find_package(elsa) (without any components), all modules are now loaded
8. Install elsa headers into ${CMAKE_INSTALL_INCLUDEDIR}/elsa/ (before /elsa/ was missing and you littered /usr/local/include, for example)
9. Add custom build target "examples" to build all examples
10. Add custom build target "elsa" to build all library modules