fmt in TimeFormatter is now changeable
`datetime.time` objects are useful as index values, e.g. in a data frame
containing data of an experiment lasting a few minutes or hours. When
plotting such data frame, the x-axis ticks are formatted as `'%H:%M:%S'`
as standard. In most cases, either `'%H:%M'` or `'%M:%S'` contains enough
information, and the `'%H:%M:%S'` format is superfluous.

The xticks were formatted based on the fmt variable in the `__call__` method. In this commit, the tick format is saved in the `TimeFormatter.fmt`
variable. This is set in the `__init__` method to `'%H:%M:%S'`, but can
later be changed, e.g.:

```
ax = df.plot()
ax.xaxis.get_major_formatter().fmt = '%M:%S'
```

is this meant to solve a particular issue? which one?

what is the purpose here?
 pls comment on the purpose.
 I think I explained the purpose in my initial comment.

The purpose is to be able to change the format used by the TimeFormatter class for formatting time when plotting time data. Currently this is only possible by somehow providing another formatting class, which is cumbersome. Due to this change, it is possible to provide a new format using just one line of code.
 pls show a complete example, which would form the basis of a tests then.
