Add `repr` to the `preprocessor` object

[geek-yang]

Following the suggestions from @BSchilperoort in PR #152, we need a nice repr representation of preprocessor object, which informs the user the setup of preprocessor.

[BSchilperoort]

It would also be nice (and more complete) to expose the stored parameters as @propertys. E.g.:

@property
def detrend(self):
    return self._detrend

etc. for the other parameters. This makes it easier to inspect this item later, for users who are for example working in a notebook.

We could also open an issue for this, and then also add a nicer repr which tells you the settings (and if the preprocessor is fit already, with an is_fit property).