feat: implement read/write graphs from/to string

[schochastics]

Fix #543

devtools::load_all("~/git/R_packages/rigraph")
#> ℹ Loading igraph
g <- make_ring(10)
write_graph(g, file = "", format = "graphml")
#> <?xml version="1.0" encoding="UTF-8"?>
#> <graphml xmlns="http://graphml.graphdrawing.org/xmlns"
#>          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
#>          xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns
#>          http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
#> <!-- Created by igraph -->
#>   <key id="g_name" for="graph" attr.name="name" attr.type="string"/>
#>   <key id="g_mutual" for="graph" attr.name="mutual" attr.type="boolean"/>
#>   <key id="g_circular" for="graph" attr.name="circular" attr.type="boolean"/>
#>   <graph id="G" edgedefault="undirected">
#>     <data key="g_name">Ring graph</data>
#>     <data key="g_mutual">false</data>
#>     <data key="g_circular">true</data>
#>     <node id="n0">
#>     </node>
#>     <node id="n1">
#>     </node>
#>     <node id="n2">
#>     </node>
#>     <node id="n3">
#>     </node>
#>     <node id="n4">
#>     </node>
#>     <node id="n5">
#>     </node>
#>     <node id="n6">
#>     </node>
#>     <node id="n7">
#>     </node>
#>     <node id="n8">
#>     </node>
#>     <node id="n9">
#>     </node>
#>     <edge source="n0" target="n1">
#>     </edge>
#>     <edge source="n1" target="n2">
#>     </edge>
#>     <edge source="n2" target="n3">
#>     </edge>
#>     <edge source="n3" target="n4">
#>     </edge>
#>     <edge source="n4" target="n5">
#>     </edge>
#>     <edge source="n5" target="n6">
#>     </edge>
#>     <edge source="n6" target="n7">
#>     </edge>
#>     <edge source="n7" target="n8">
#>     </edge>
#>     <edge source="n8" target="n9">
#>     </edge>
#>     <edge source="n0" target="n9">
#>     </edge>
#>   </graph>
#> </graphml>


s1 <- r"---(<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns
         http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
<!-- Created by igraph -->
  <key id="g_name" for="graph" attr.name="name" attr.type="string"/>
  <key id="g_mode" for="graph" attr.name="mode" attr.type="string"/>
  <key id="g_center" for="graph" attr.name="center" attr.type="double"/>
  <graph id="G" edgedefault="directed">
    <data key="g_name">In-star</data>
    <data key="g_mode">in</data>
    <data key="g_center">1</data>
    <node id="n0">
    </node>
    <node id="n1">
    </node>
    <edge source="n1" target="n0">
    </edge>
  </graph>
</graphml>
)---"
g1 <- read_graph(s1, format = "graphml")
g1
#> IGRAPH c88e052 D--- 2 1 -- In-star
#> + attr: name (g/c), mode (g/c), center (g/n), id (v/c)
#> + edge from c88e052:
#> [1] 2->1

g <- make_ring(10)
s1 <- r"---(0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 0 9)---"
g1 <- read_graph(s1, format = "edgelist", directed = FALSE)
isomorphic(g, g1)
#> [1] TRUE

^{Created on 2025-07-09 with reprex v2.1.1}

cc @ntamas @szhorvat @clpippel since you all were discussing this. I hope I understood the issue correctly and the implemented solution makes sense