ENH: remove as_staypoints calls from location_identification

[bifbof]

closes #476
This PR removes the as_staypoints assertions, as the staypoints need to be created initially and there this property is cached.
It doesn't really solve the verify problem, but my argument is that we require a GeoDataFrame (as trackintel staypoints) and thus can rely that we get the right data shape.
In all other places as_xyz works as intended, since it is always applied to newly created objects and thus is not yet cached.
However, I would like to make this a bit clearer with a proper class hierarchy (which is backwards compatible with our current as_xyz approach), but we should discuss that elsewhere.

[hongyeehh]

Thanks. I agree that we shall not check for validity, as we already stated in the documentation. I will merge this PR now.