Allow for importing configuration files

[fxnn]

Within a configuration file, it should be possible to import other configuration files by using an import statement on the top level.

Syntax should be as follows. At first, we should be able to import a single file.

import: /path/to/import.yml

Then, we should be able to import multiple files at once.

import:
- /path/to/import1.yml
- /path/to/import2.yml

The semantics shall be as follows. On importing a configuration file, every logfile defined therein must be added to the logfiles defined in the importing configuration file. Equally, every summary defined in the imported configuration file must be added to those defined in the importing configuration file.