Move plotting code out of data package creation script

[simleo]

The current version of the script that creates data packages also generates a number of plots. To do this, it uses the objects, links and tracks dataframes returned by pushtopandas.push_to_pandas. While the objects and the links dataframes are initially generate by the readers, the the logic that generates the tracks dataframe is currently embedded in pushtopandas.push_to_pandas. Moreover, we currently don't have a reader for our datapackage format. For these reasons, it's currently not straightforward to generate the plots starting from a datapackage directory.

• Move tracks dict creation code out of push_to_pandas
• Add a biotracks datapackage reader
• Move plotting code out of create_dpkg.py