Add more methods and Refactor code
Add more methods and Refactor code

Constructor now accept first argument as paragraph parent. Also create a reference from its parent.

parseWords now pass a sentence rather than sentenceId.

parent() method get paragraph parent from a sentence.

This commit also introduce lookup methods, they are:
- findWordById = find a word by its id
- findWordByIndex = find a word by its index

Sentence has navigate method too:
- next() is for navigating to the next sentence in a paragraph context.
- prev() is for navigating to the previous sentence in a paragraph context.
raitucarp/paratree