Fix FixedFormatter

[StefRe]

• use FixedLocator with FixedFormatter and change labels to something
  more appropriate for a FixedFormatter
• add docstrings to pass flake8

The FixedFormatter example was completely wrong with misplaced minor tick labels (due to using FixedFormatter with MultipleLocator). The minor tick labels are a bit confusing as it's not shown how they were created, so I removed them altogether.

Current:

Fixed:

[rougier]

Thansk, that might explain why I needed the first empty label. Since you fixed it, maybe you use zero instead of the empty string.

[StefRe]

maybe you use zero instead of the empty string

done