There are 4 possible cell types:
- Markdown
- Heading
- Raw
- Code

Markdown cell type consists of:
- metadata
- Source <- List of Strings

Heading cell type consists of:
- metadata
- Source <- List of Strings
- level <- int

Raw cell type consists of:
- metadata

Code cell type consists of:
- metadata
- collapsed <- Boolean
- input <- List of Strings
- language <- String
- prompt_number <- int
- outputs <- List of Outputs

Output of Code cell consists of:
- metadata
- output_type <- String
- text <- String
- one of [png, latex, stream, ..] <- String
- prompt_number <- int     *present only with latex field


Cells are packed into worksheets.