Enable download of datapackage as zip

[Bachibouzouk]

This enable the download of a datapackage of a scenario.

One can use the URL:
http://127.0.0.1:8000/en/scenario/export/datapackage/

Or click on "Export as datapackage" on the progession ribbon (this is just for convenience, we should have this button somewhere else)

TODO:

• Generate the datapackage.json automatically as well
• Get the foreign keys of datapackage.json correctly
• Figure out a parameter matching between open_plan parameters and the placades parameters --> this is done in Add components from openPlan tool placades#5

[Bachibouzouk]

The idea is to enable download of datapackage and then iterate to make changes to the to_datapackage method

[paulapreuss]

Using the current pinned version of tabular in the requirements (pointing to the dev branch), I get the error infer_metadata_from_data() got an unexpected keyword argument 'fk_targets'. I looked around the oemof-tabular repo and saw your PR which introduces this keyword in fix/datapackage. However, pointing to this branch instead causes the following dependency conflict in the requirements:

ERROR: Cannot install datapackage==1.5.1 and jsonschema==4.4.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested jsonschema==4.4.0
    datapackage 1.5.1 depends on jsonschema<3.0 and >=2.5

I was able to bypass this error by unpinning the jsonschema version, but now I am not sure if the version installed might cause issues within the functions that use the package. Would have to double check this.

After this, the datapackage export seems to work and contain all the necessary files. Very cool feature!