Add distribution and category stati

[Lillliant]

I've made a few distributions for aspects and words (tokens) under these names:

• nreviews_naspects: number of reviews that has 1 aspect, 2 aspects, ...
• nreviews_ntokens: number of reviews that has 1 token, 2 tokens, ...
• naspects_nreviews: number of aspects appearing in 1 review, 2 reviews, ...
• ntokens_nreviews: number of tokens appearing in 1 review, 2 reviews, ...
• ncategory_nreviews: aspect category of the semeval dataset and their respective number of reviews