message: Remove the modules graph.
diff: The Ginkgo library can be grouped into [modules](modules.html) and these modules form the basic building blocks of Ginkgo. The modules can be summarized as follows:
  -\dotfile modules.dot
  -
  * @ref Executor : Where do you want your code to be executed ?
  * @ref LinOp : What kind of operation do you want Ginkgo to perform ?
  * @ref solvers : Solve a linear system for a given matrix.
  