Permutation.cpp: Added a class with methods for iterating over all Permutations
in a given S_n
Permutation.cpp: Methods that return Permutation objects now set their `even`,
`order`, and `lehmer` values whenever they are easily derivable
Like the last commit, but for Python
Translated lehmer02.py to C++
Gave permutation_gen::iterator an operator->()
Permutation.cpp: Combined the shared parts of permutation_gen::iterator's +=
and -= operators
Completed the "throw" statements in Direct.cpp and Group.hpp