Change the way Features request statistics

[Ayush Tomar]

At present, Feature::Internal class does the work of calculating statistics like normalised lengths, query term frequencies, etc,. Feature subclasses then request these statistics from get_value() method. Due to presence of Internal member in Feature class, subclassing Feature is not easy as well.

The following changes need to be made:

1. Feature::Internal should be eliminated and its functionality (i.e. calculating and supplying statistics to Feature subclasses) should be made part of FeatureList class (probably as FeatureList::Internal class).
   

2. It's possible that we could also do the thing Weight does, and have each Feature subclass report which statistics it needs, so FeatureList can calculate them each once in advance before calling get_value() on all its Feature objects.

[Ayush Pandey]

Hi ............ wanted to Know if the said changes were made or can I claim this ticket and start working on it

[Olly Betts]

Setting milestone to the release this should debut in.