Modified requirements.txt and conf.py to use sphinx_bootstrap_theme.
[ref-231] Removed factors and cardinalities with all corresponding methods from
UndirectedGraph model.
[ref-231] Refactored MarkovModel class to have all the methods for factor
operations and cardinalities of the variables.
- Initial commit for tests of FactorGraph class and methods
- Refactoring in FactorGraph to have all the factor related operations over
  here.
- Initial commit for tests of JunctionTree classes and methods
- Refactoring in JunctionTree to have all the factor operation over here.