Move seed parameter from Model and RegParser to Settings and SettingsBuilder

Currently, it is not possible (to my knowledge) to configure the model seed directly from a parser.

I propose moving the seed parameter from `Model` and `RegParser` to `Settings` and `SettingsBuilder` so that it can be configured through parser options and centralized alongside other solver settings.

A minimal implementation could look like:

`Settings.java`
```java

protected long seed;

public void setSeed(long seed) {
this.seed = seed;
}

public long getSeed() {
return seed;
}

```
`SettingsBuilder.java`
``` java

public static final String SEED = "seed";

@Option(name = "-seed", usage = "Set the seed for the random number generator.")
protected long seed = 1_000_000_007L;

```
`Model.java`
``` java

public void setSeed(long seed) {
settings.setSeed(seed);
}

public long getSeed() {
return settings.getSeed();
}
```