Change anchored_artists.AnchoredSizeBar: docstring, vertical size, color

- Add size_vertical variable, which sets the height of the size bar.
- Add color argument, which change the color of rectangle bar (not the
  label, need to figure out how to do that).
- Docstring: listing the possible arguments and adding an example.
Change anchored_artists.AnchoredSizeBar: put label over rectangle

Add variable label_top to change the position of the label from below
the rectangle to over the rectangle
Merge pull request #2158 from magnunor/anchored_size_bar_height

Changes to anchored_artists.AnchoredSizeBar