Moved self.frames initialization of inner lists to `__init__`
That allowed update to be simplified
`flotify_unput` added to `update` to transform low_level_data into all floats and make it the frame
`initial_value` in _predict no longer optional
  only place calling it always provided value
Tests updated to reflect changes by adding, subtracting, changing
Tests added to ensure coverage where it was lacking
New testing technique:
  `super()` calls mockable
    obtain the parent class object (this is what `super()` returns already)
    patch any calls to it, including `__init__`

Changed docstrings to Numpy format
Added print_msg from util for no raise error output
Enforced window size > 2, otherise raise error
Black and Pylint changes
  no single quotes for comments or strings
  use enumerate not range and len