changed construction for Formulas representing variables
capitalized the names of the constants ONE and ZERO
added constants for the boolean functions
default constructor for Pairs removed
modified some names for boolean functions
added a datatype for variable orderings
added getter and setter for the list inside the variable ordering type
changed the type of the OBDDs variable ordering to VariableOrdering
started with the apply function (currently commented out)
removed remaining unused getters and setters
corrected setter for the VariableOrderings list
added function that provides the Formula represented by an OBDD
added indexOf function for VariableOrdering data type
added functions Value for evaluation and Number (of satisfying assignments)