Make Model::operator() const

[FabianThomsen]

As Model::operator() doesn't change the model itself, it should be declared const.
For this to work, we cannot use std::unordererd_map::operator[](Key & key), inside it, because that would insert a new value if, the key didn't exist. As alternative, we can use std::unordered_map::find(Key & key), which would throw if the key didn't exist - a behavior that also seems more intuitive to me.