Better metafeatures_to_compute hyper-parameter

[mitar]

I think that instead of metafeatures_to_compute = hyperparams.Hyperparameter[List[str]] as a hyper-parameter, it would be better to list all metafeatures explicitly. Something like:

metafeatures_to_compute = hyperparams.Set(
    elements=hyperparams.Enumeration(
        values=['name1', 'name2', 'name3', ...],
        # Default is ignored.
        # TODO: Remove default. See: https://gitlab.com/datadrivendiscovery/d3m/issues/141
        default='',
    ),
    default=('name1', 'name2', 'name3', ...),
    semantic_types=['https://metadata.datadrivendiscovery.org/types/MetafeatureParameter'],
    description='Custom list of specific metafeatures to compute by name.  Only used if \'metafeature_subset\' hyperparam is set to \'CUSTOM\'',
)

Moreover, you can also use a Choice hyper-parameter, which would all you to change metafeature_subset so that it is a choice between multiple different options, and if custom is picked, it has a sub-hyper-parmeter metafeatures_to_compute.