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

Merge pull request #53 from Schoyen/dim-property

Dim property

Merge pull request #51 from Schoyen/Schoyen-patch-1

Use Python 3.8 when testing on Travis

Merge pull request #49 from Schoyen/spin-tests

Spin fix

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

Custom one-body operator

Merge pull request #38 from Schoyen/sparse_dvr

Sparse dvr

Merge pull request #47 from Schoyen/fixes

Minor bugfixes

Merge pull request #45 from Schoyen/spin-matrices

Spin matrices

Bug fixes

Include required packages in installation