Add resize function on PrimitiveGroup

[anderscognite]

After a group is full / a new one is created, we may want to resize the current group so its capacity is exactly its count to minimize memory usage. This will require some extra copying which hopefully isn't much slower.

[Verkhovskaya]

This is unnecessary with the new file parser - I use sector metadata to make sure the groups I create are exactly the right size.

[anderscognite]

If we’re keeping protobuf for a long time, then we might have to, but agree on the new parser:)

[anderscognite]

I'll close this as protobuf is dying!