Replace init_network() part of interface

[FrederikHeber]

init_network() is quite clumsy and the setup parameter is ont very intuitive.

Maybe we could change this into an initial init_network() (which might be hidden in some general init() function or even in the constructor of the class) and then have functions like add_feature("sample") that would add a specific feature to the computational graph by adding its nodes. This could be explained by the extra cost that some features (hessians) incur on construction.

Functions in the simulations interface should also check whether these features are present and give appropriate error messages suggesting to add the respective feature.