WCSAxes Major Formatter Does Not Update When Format Unit is Changed

### Description

The WCSAxes Major Formatter Does Not Update When Format Unit is Changed.

### Expected behavior

Expected behaviour is that the Major formatter updates itself such that an appropriate precision is used by on the units.

### Actual behavior

The above does not happen. In the below example, the y axis is stored in the WCS as metres and then changed to nm for the plot. However, as can be seen the precision is kept leading to many extraneous 0s.

Start by locating the WCSAxes major formatter and the code path that handles changing a format unit. Reproduce the reported metres-to-nanometres y-axis example and compare the displayed precision before and after the change. Done means the formatter recalculates precision for the new unit without extraneous zeros.

Written by the indexing model from the issue text.