Add properties: is_fork and parent

[alextanski]

While working on streamlining DP workflows I was looking for reliable ways to tell a dataset fork from its main/parent dataset (or the upstream dataset in Crunch terms) while also being able to return the latter one's id when working with it's fork.

I've already confirmed with Alessandro that we can use the dataset/{id}/actions/upstream_delta catalog (see https://docs.crunch.io/feature-guide/feature-versioning.html?highlight=fork), leading to two handy methods (should be properties if added to the scrunch dataset object):

To return the parent id if we are working on a fork:

def parent(dataset):
    """
    Return the parent's (upstream) dataset id if the passed dataset is a fork. 
    """
    try:
        upstream = dataset.resource.actions.upstream_delta
    except pycrunch.lemonpy.ClientError:
        return None
    return upstream.value.upstream.id

To simply test if we are working on a fork:

def is_fork(dataset):
    """
    Test if the current dataset is a fork.
    """
    return parent(dataset) is not None

Adding these two would help greatly. What do you think?