Check if input is as expected and throw meaningful errors.

[weidler]

Especially when using batches in the networks, there can be easy mistakes be made when feeding input. We should therefore at crucial places add checks that test whether the given format is what the following code expects. E.g. when feeding networks one can give a one-dimensional input. This should be prohibited if the network wants to deal with batches.

[HansBambel]

Done with #23?