add in-place `generate_offsets` function

[xjrc]

There are a number of places where the conduit::blueprint::mesh::topology::unstructured::generate_offsets function is used to generate offsets in their default path (i.e. for input topology n, at path n["elements/offsets"]). In order to reduce the technical burden of pre-generating these offsets for internal code and savvy users, a non-const, in-situ alternative should be provided with an interface like conduit::blueprint::mesh::topology::unstructured::generate_offsets(Node &n).