Provides a logical value on whether the graph is property graph (i.e., all
nodes have an assigned `type`

value and all edges have an assigned `rel`

value).

## Examples

```
# Create a graph with 2 nodes
# (with `type` values) and a
# single edge (with a `rel`)
simple_property_graph <-
create_graph() %>%
add_node(
type = "a",
label = "first") %>%
add_node(
type = "b",
label = "second") %>%
add_edge(
from = "first",
to = "second",
rel = "rel_1")
# This is indeed a property graph
# but to confirm this, use the
# `is_property_graph()` function
is_property_graph(simple_property_graph)
#> [1] TRUE
# If a `type` attribute is
# removed, then this graph will
# no longer be a property graph
simple_property_graph %>%
set_node_attrs(
node_attr = type,
values = NA,
nodes = 1) %>%
is_property_graph()
#> [1] FALSE
# An empty graph will return FALSE
create_graph() %>%
is_property_graph()
#> [1] FALSE
```