Howmany

- Updated the name of the unit function to make it more succinct.
- Updated documentation to be more succinct and understandable.

- The user can now pass predefined amounts, labels and units to `howmany.compare()`.
- Default values were removed from `howmany.compare()`.
- The example Jupyter notebook and examples in the readme were expanded to show v0.2.0 functionality.
- Minor bugs were fixed for unit conversions and checking whether units are `None`.

- The main function `howmany.compare()` has Wikidata QIDs and properties passed to it and returns a dictionary of labels and the ratio between the property values.
- Labels can be returned in any language given the `iso` argument of `howmany.compare()`.
- Allows for comparing area that are in square kilometers as well as meters via length and width properties.
- Basic unit and dimension conversions have been added.
- Wikidata REST API functions have been made for easy data statement access.
- A utility function has been added to convert floats to strings so people can more easily display how small something is in comparison to another.