remove the final modifier to ContentFeatureStore.addFeatures method.

ContentFeatureStore.addFeatures works by grabbing a FeatureWriter from
the concrete subclass, but it can be the case that that's not the best
way of doing so for a given immplementation, so removing the final modifier
from the method allows for further optimizations on the subclasses where
appropriate while keeping the default behavior.
Merge pull request #115 from groldan/contentfeaturestore_addfeatures

remove the final modifier to ContentFeatureStore.addFeatures method.