Base.similar method for acsets

[KevinDCarlson]

There are some places in the code (for instance in the nauty integration, but I bet not just there) where we copy or even deepcopy a whole acset just to get an acset of the same shape that we'll then overwrite. It might be nice to write a method of Base.similar that copies an acset's shape (i.e. attrtypes and cardinalities of homs) but leaves everything uninitialized for this kind of situation. I don't know enough about columns to know whether this would be trivial or annoying.

[epatters]

Good idea. It should be straightforward to implement this feature without reaching into the internals. Just create an empty acset of the same type (I think/hope Base.empty is already implemented for acsets) and call add_parts! without setting any subparts.