Fixes Issue #79. Fixed repetition of text in to_text and to_html methods of the Document class

Fixes #79 Here is the current implementation of the `to_text` method: ```python def to_text(self): """ Returns text of a document by iterating through all the sections '\n' """ text = "" for section in self.sections(): text = text + section.to_text(include_children=True, recurse=True) + "\n" return text ``` The issue occurs when the document tree has the following (or similar) structure: ``` Section-1 ├── Section-2 ├── Section-3 ``` The text of `Section-2` is included in the output when the `to_text` method is called (recursively) for `Section-1` as well as for`Section-2`. Similarly, the text of `Section-3` is also duplicated in the output. To remove the duplicates, iterate over all the sections in the document and choose the top sections i.e., those sections which are not a children of any other section. At the end, concatenate the (recursive) text of all the top sections. Here is a summary of the changes made: - The method `_get_top_sections` returns all the top sections in the document tree. - `to_text` and `to_html` methods accept a boolean parameter `include_duplicates`. - If `include_duplicates` is `False`, only the top sections are considered. - Otherwise, all the sections are considered. You can take a look at this fix in **Google Colab** [here](https://colab.research.google.com/drive/1YatMfrkFxJYfB6Ohkml9PGhZTnTng3-i?usp=sharing).