Unyts

Fixed error due to a not-declared message variable when starting the program from a clean installation.

Some improvements in this release:

- Now it is possible to change the _formation volume factor_ and the _density_ constants_ from the GUI.
- The *lean_BFS* is the default algorithm.
- The conversion is shown by default in the GUI. Can be activated/deactivated from the Options menu.
- The pressure unit _kg/cm²_ is linked to _absolute bar_ to convert from pressure units to weight/area ratio.
- International System prefix _quetta_ **Q**, _ronna_ **R**, _ronto_ **r**, _deca_ **da**, _quecto_ **q** are now included.
- To avoid confusion between _reservoir cubic metre_ and _ronto cubic metre_, the abbreviation will **rm3** will be used for _ronto cubic metre_ and **rem3** for _reservoir cubic metre_.
- The units _parsec_, _astronomical unit_ **au** and _light year_ are included (mostly for fun).


A new search algorithm, **lean BFS**, was implemented, which leans the graph before giving it to the BFS search.
Improved the Graphical User Interface, options are available from menus and the conversion path is shown in the GUI.

Implementing GUI.
Install version 0.7.1 and then execute `python -m unyts` or double-click the shortcut here available.

Simplified code to make it **compatible with Python 3.7** and subsequent. The previous release will fail on Python below 3.10.

The interaction with other packages like Numpy, Pandas and Cloudpicker has been modified to make them optional, avoiding the required dependence.

Several revisions and improvements:
- cleaned dictionary,
- reviewed digraph class (for network conversion),
- added more subclasses units,
- revisited testings.

latest
The ratios of _weight_ to _area_ are now defined as pressure units, related by **_1_ absolute bar** = **_10/Earth gravity_ kg/cm2**.