New features: from_package() and add_entity()

[lapidus]

@miroli, regarding new functionality to f2p, this can provide some inspiration when it comes to the section of “ingredients”:
https://ddf-utils.readthedocs.io/en/latest/recipe.html

cooking:
    concepts:
        # procedures for concepts here
    entities:
        # procedures for entities here
    datapoints:
        # procedures for datapoints here

But at the same time I think good that f2p is lean and focused on the most common workflows we see from Pandas.

I guess what I'm primarily looking for is a way to map more quickly define the basics (what was previously in the constructor of f2p) of a dataset.

For example, let's say I already have a dataset and would like to just add one more file, pseudo:

f2p.from_package("path")
f2p.add_data(data=df, concepts=concepts)
f2p.to_package("path")

Or my simpler use case where I would like to create a dataset with some concepts and entities (no datapoints):

concepts = [
        {
                'concept': 'country',
                'concept_type': 'entity_domain'
        },
        {
                'concept': 'some-other-country-metadata',
                'concept_type': 'String'
       }
]

f2p()
f2p.add_concepts(concepts=concepts)
f2p.add_entity(concept="country", data=dataFrame)
f2p.to_package("path")

[miroli]

I will look into updating an existing package. As to the second request, I believe this functionality is already available. The update_entity method can accept non-existing entities, and simply adds them to the entities list if that's the case.

See the following example.

concepts = [
        {
                'concept': 'country',
                'concept_type': 'entity_domain'
        },
        {
                'concept': 'capital',
                'concept_type': 'string'
       }
]

df = pd.DataFrame([
    {
        'country': 'sweden',
        'capital': 'Stockholm'
    },
    {
        'country': 'norway',
        'capital': 'Oslo'
    },
])

f2p = Frame2Package()
f2p.add_concepts(concepts=concepts)
f2p.update_entity(name='country', data=df, id='country')
f2p.to_package('ddf')

[miroli]

As to the first point, I'm thinking it would probably be sufficient to add a .append_to_package method that:

1. Adds any new concepts to ddf--concepts.csv.
2. Adds one or more data files if they don't yet exist in the package.
3. Adds new entity files or appends to existing ones.
4. Recreates the JSON schema and updates datapackage.json.

Do you think that would do it @lapidus?