Merge pull request #1 from eunjongkim/master

fixed bug in series expansion
Merge pull request #2 from eunjongkim/master

fixed a bug in series expansion
reran sympy pauli operator test notebook
Merge branch 'master' of github.com:jrjohansson/sympy-quantum-notebooks
reran sympy pauli operator test notebook
added initial version of OperatorFunction
added robert's revision of file: OperatorFunction Class