Add more functionality to `database.echemdb.Echemdb`

[DunklesArchipel]

The ' cv.cv_collection' (database.echemdb.Echemdb, see #98) could be extended with the following functionality.

• save: saves the current entries of, e.g., a filtered collection as datapackages into a specific folder.
• load: load datapackages from a source, e.g., produced by save.
• plot: plot all entries of that database in a single plot (maybe ask if the number of entries exceeds ten). Allow direct comparson of CVs.
• materials: list of materials used (Added in Add describe to cv.database echemdb#20 )
• material: (from Make code to generate website accessible to search echemdb echemdb#23 ) Return filter() with a predicate that filters by electrodes.counterelectrode.material==value. Not sure what is relevant in practice but we should provide all kinds of methods like this for what we and others typically want to search for.
• describe: Statistics about the database. partly addressed in Add describe to cv.database echemdb#20. More discussion in Add more information to the database describe function #6 .