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