Positive z down- or upwards

• New examples:
  • empymod can handle positive z down- or upwards (left-handed or right-handed coordinate systems; it was always possible, but not known nor documented). Adjusted documentation, docstrings, and added an example.
  • Example how to calculate the responses for the WalkTEM system.
• Minor things and bug fixes:
  • Change from relative to absolute imports.
  • Simplified releasing (no badges).
  • Python 3.8 is tested.
  • Fix: numpy now throws an error if the third argument of logspace is not an int, some casting was therefore necessary within the code.