Rework File IO

This MR reworks and unifies the input and output of files and paths. The following changes have been made:

• ModuleManager and central Corryvreckan class are now independent (more of a structural than functional change)
• All input paths are now taken relative to the configuration file. In modules, use config.getPath("myfile") instead of config.get<std::string>("myfile) as the latter will again be relative to the executing folder, not to the configuration file.
• There are three new global parameters output_directory, deny_overwrite, purge_output_directory, described in the manual.
• Output files created my modules should use

  auto path = createOutputFile(config.get<std::string>("output_file_name"));
  auto my_file = std::ofstream(path);

  in order to end up in the configured output directory
• The global configuration is not anymore inherited by individual modules. This was very messy anyway.

This fixes #17 (closed)