v0.2.7: Multiple time evolution operators (#57)

* Delete __init__.py in tests-directory

* Return self when passing in system

* Allow for list of time-evolution operators

  This lets the user set up multiple (independent) time-evolution
  operators and sum these together. An example might be a time-dependent
  potential and a dipole laser acting on a system.

* Add test of empty operators

* Update documentation

* Add documentation to set_time_evolution_operator