Assert the number of crops that have a value for all practicalplants properties

[petteripitkanen]

Currently the number of crops that are missing a value is asserted only for the properties that are extracted. Add a test to assert for each practicalplants property the number of crops that have a value (instead of missing a value) in the static dataset. This should use directly the JS file for the static dataset instead of using the function readCropsLower which is deprecated and will be removed later.

This will also be useful for finding an explicit list of little used properties for #74.