Drop is translated into a Projection which creates issues for inconsistent partitions

[phofl]

see test_drop_meta_mismatch in dask/dask

drop currently is translated into a Projection which is based on meta, so if some partition don't have some of those columns then you will run into KeyErrors, this can potentially be problematic.

A potential workaround is to create it's own Expression class and let it inherit from Projection so that it is taken into account in all optimisations, I have to think a little bit more about this if this is really a good idea