Junctiontree

- Fix crashing of factor graphs without edges.

- Format README.

- Fix README format.

- Fix long description content type for PyPI.

- Unify language for keys and variables to use the term "variable".
- Re-factor beliefpropogation code into a computation and construction module.
- Use Shafer-Shenoy updates for belief propogation in junction tree.
- Remove usage of networkx library in triangulation tests.
- Remove usage of list concatenation via sum() function.

- Fix support for duplicate factors (2).
- Assert that factors are given as a list of lists.