Add CI System to Sassena and Modernize Build-System

Add a CI System to Sassena. Currently it builds the code as well as the documentation (including option to publish to the gitlab pages site) and runs a couple of unit tests. This MR also includes changes to the CMake scripts so that the project is up-to-date with modern practices and removes a lot of the old cruft. Another important change is moving the many miniature static libraries (e.g. log, which only has one translation unit) into one library called SassenaCore (under /src/core). This library is then linked with an application (under src/app) to provide the command-line functionality for the user.