Pr/bugfixes for PT-TEBD

[gefux]

Fixes #69 and #70, including test.

• The contribution has been discussed and agreed on in the Issue section.
• Code contributions do its best to follow the zen of python.
• The automated test are all positive:
  • tox -e py36 (to run pytest) the code tests.
  • tox -e style (to run pylint) the code style tests.
  • tox -e docs (to run sphinx) generate the documentation.
• Added test for changed/added code.
• API code contributions include input checks (defensive code).
• API code contributions include helpful error messages.
• The documentation has been updated:
  • docstring for all new functions/methods/classes/modules.
  • consistent style of all docstrings.
  • for new modules: /docs/pages/modules.rst has been updated.
  • for api contributions: /docs/pages/api.rst has been updated.
  • for api contributions: tutorials and examples have been updated.