Make the Backend machine-agnostic

[MichaelHirn]

At the moment you need to specify, which Framework and which Hardware you would like to use for your Backend (implying for your computation).

To make Collenchyma (and therefore the application using Collenchyma) adapt to the underlying hardware, we need to extend the Backend with an Option<BackendConfig> as initialization argument. If no BackendConfig is provided, the Backend should check on each Framework, load the hardware available and make an educated guess about which Framework + Hardware to select for computation.