Refactor data source for GenKwConfig

[xjules]

To be able to distinguish between the source of data when instantiating GenKwConfig we need to speicify either it's sample_prior, forward_init or design_matrix. Currently we rely on setting some flags in GenKwConfig; nevertheless this causes a lots of unnecessary if statements to check what the GenKw really is. Therefore we should implement each sub init_source as @DataClass instead.
For instance:

@dataclass
class DesignMatrix:
    name: Literal['design_matrix']= 'design_matrix'
    
@dataclass
class SamplePrior:
    name: Literal['sample_prior']= 'sample_prior'
    
@dataclass
class ForwardInit:
    name: Literal['init_file']= 'init_file'
    init_file: str

@dataclass
class GenKwConfig(ParameterConfig):
   ....
    init_source: Union[SamplePrior, ForwardInit, DesignMatrix] = SamplePrior()

[xjules]

Closing this one as it is superseded by #10095