Raise when overriding a CRS via crs property setter

[martinfleis]

When using set_crs, we raise a ValueError when the GeoSeries already has a CRS which is not equal to the passed CRS. But when assigning directly to .crs as gdf.crs = 4326, we just let user to override the existing CRS, which is a quite common error.

I propose to add checks to crs property setter and allow assignment only when current crs is None (or equal but None feels safer) and point users either to set_crs(crs, allow_override=True) if they really want to override it or to to_crs(crs), which in most cases will be what that actually wanted to do.

This will probably require a deprecation period where we first warn and then raise in a subsequent release. It would be nice to make that switch to raise in 1.0 but I don't think there's 0.15 coming in the meantime, so let's start with a warning.

[m-richards]

I just had a quick look at #3088, @martinfleis I think you mean that we should do this for both GeoSeries.crs and GeoDataFrame.crs, correct? (that PR is currently just GeoSeries)

[martinfleis]

Yes, we should surely do it for both.

[jorisvandenbossche]

The merged PR in the end also did it for GeoDataFrame, so we can close this issue (I started doing a PR for finishing this, but so then only updating the changelog -> #3333)

[jorisvandenbossche]

Closed by #3088