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

v0.2.6: Merge pull request #53 from Schoyen/dim-property

Dim property

New naming convention

Rename dipole-interaction operator and the dipole-moment integrals.

Spin-fix

This release addresses several bugs in the implementation of the spin-squared operator.

Custom one-body operator

Merge pull request #48 from Schoyen/custom-one-body-operator

Custom one-body operator

Sinc-DVR

Inclusion of latest Sinc-DVR changes from the thesis of @halvarsu.

Bugfixes

Merge pull request #47 from Schoyen/fixes

Minor bugfixes

Spin operators

This release include matrix elements for the one- and two-body spin operators in the GeneralOrbitalSystem.