Remove Module and Adapter as root elements

It would be easier for users to always use the same root element and have no need to adjust the root element based on the use case. See description in the Frank!Runner README for the current three options:

https://github.com/ibissource/frank-runner#code-completion-with-frankconfigxsd

So I would like to suggest to remove the possibility to have Module and Adapter as root elements and allow a Configuration element in a Configuration element for the use case where one configuration file is included in another configuration file using an entity reference. Only one include level is enough in my opinion (no need to recursively include a Configuration in a Configuration).

This also means F!F needs to be adjusted to be able to parse a Configuration element in a Configuration element when initializing a configuration. Ask Gerrit and/or Niels to look into this when you change the Frank!Doc for this.