Restrict discrete grid codes to array indices

[timmens]

In this PR,

I remove the ability to specify general discrete grid codes. While it may be beneficial in the future, we currently do not have a use case, while the feature makes maintaining the library more complex. We have, therefore, decided to drop the feature for now. If it turns out to be needed in the future, we can easily add it by following this PR.

Additionally, I renamed the discrete deterministic test model from ISKHAKOV_ET_AL_2017_FULLY_DISCRETE to ISKHAKOV_ET_AL_2017_DISCRETE since the state is no longer discrete, only the choices.

What changed?

This was allowed before merging of this PR:

@dataclass
class Health:
   bad: int = -1
   neutral: int = 0
   good: int = 1
   
DiscreteGrid(Health)

Now, this would raise an error, and you have to specify the grid like so:

@dataclass
class Health:
   bad: int = 0
   neutral: int = 1
   good: int = 2
   
DiscreteGrid(Health)

[hmgaudecker]

Very nice -- much easier to read!!!

[hmgaudecker]

Even nicer!