Use properties instead of fields for the state struct in compiled exploration

Currently, the locations and state variables of a model are compiled as fields into the state struct type in compiled exploration (in the Exploration project). This is inflexible since the way that values are stored cannot be changed independently of how the values are accessed. We should instead implement properties for the fields and only access (get and set) them via these properties. As a second step, this will allow more advanced storage, e.g. bit-packing multiple state variables into a single (short, int, long etc.) field of the struct.