Implement as_dm.sc()

[krlmlr]

with separate methods for all models if necessary.

Only suggest dm, delayed S3 registration via vctrs::s3_register() .

Example for PATH():

x %>%
  unclass() %>%
  as_dm() %>%
  dm_add_pk(object, object_) %>%
  dm_add_pk(path, path_) %>%
  dm_add_pk(vertex, vertex_) %>%
  dm_add_fk(path, object_, object) %>%
  dm_add_fk(path_link_vertex, path_, path) %>%
  dm_add_fk(path_link_vertex, vertex_, vertex)

Result:

[mdsumner]

yes, appreciate the input - this is overdue and I hope to spend some time on it in next months