Refactoring libfovis to comply with ROS standards and creating suitable install targets

See #5

This pull request includes a bit more refactoring than I hoped for. Since ROS installs header files into an `include/${PROJECT_NAME}` directory to keep the includes unique, the behaviour between the `devel` and `install` version of this library became ambiguous. Using the install version requires includes like `#include <libfovis/fovis.hpp>` whereas the devel version would only work with `#include <fovis.hpp>`. To generate a consistent behaviour for both build targets I moved the header files to an `include` directory and the source files to a `src` directory. Now everything uses the `#include <libfovis/fovis.hpp>` syntax. Sadly, this requires some refactoring in the package relying on `libfovis` (see below PR for `fovis_ros`). As the installable debian package was not working in the first place, this is only a problem for people using libfovis as a submodule. In those cases they would have to change their includes to make it work again. Considering the benefits of having a working version on the ROS package servers I think this is a small price to pay.

Please test this PR and come back to me with any questions or suggestions. I would appreciate if this could find its way into the upstream repository because I would love to use libfovis in our project.