Add four simple layouts

[Chriscrosser3310]

Related to #280. Four simplest layouts were added: bipartite, circular, shell, and spiral. A few things to note:

1. The rescale and recenter functions are duplicated with the ones in Add spring layout #306, we only need to keep one of each.
2. The layout functions return the new type Pos2DMapping added in Add random_layout function and Pos2DMapping #305.
3. The tests are performed as follows: generate a position dictionary from a layout function in networkx, and compare it to that generated by retworkx with the same layout function and inputs. If for all nodes, the difference between positions of the same nodes is smaller than some threshold (I chose 1E-6), the tests pass.

[coveralls]

Pull Request Test Coverage Report for Build 893561305

• 278 of 282 (98.58%) changed or added relevant lines in 2 files are covered.
• 18 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.09%) to 96.865%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/layout.rs	178	182	97.8%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	18	99.32%

Totals
Change from base Build 888019140:	1.09%
Covered Lines:	8714
Relevant Lines:	8996

---

💛 - Coveralls

[georgios-ts]

Great work!
My only comment is that you should consider updating all layout functions to handle graphs with holes in node indices.

[Chriscrosser3310]

Great work!
My only comment is that you should consider updating all layout functions to handle graphs with holes in node indices.

Thanks, this is a good point, I forgot that indices can be non-consecutive. Will update soon

[georgios-ts]

This

n = 5
graph = rx.generators.mesh_graph(n)
graph.remove_node(2)
rx.shell_layout(graph, nlist=[graph.node_indexes()])

will raise a panick exception.
I suggested some changes to fix this but feel free to proceed in your own way.

[georgios-ts]

Great! One comment left and then it should be ready to go

[georgios-ts]

Suggested change

	if let Some(scale) = scale {
	rescale(&mut pos, scale, (0..node_num).collect());
	}
	}
	}
	if let Some(scale) = scale {
	rescale(&mut pos, scale, (0..node_num).collect());
	}

[Chriscrosser3310]

Nice catch, I forgot there's an else if -- but the problem of moving it out is that if the graph only has 1 node, lim in rescale will be 0.0 and the point will become [NaN, NaN]. I guess I can modify rescale so it only does something when lim > 0.0.

[mtreinish]

Overall this LGTM, great work! I have 2 nits inline but not really worth blocking over. I might just update the test one myself real quickly so we can merge this (because getting good error messages from the tests are useful).

[mtreinish]

Personally I would do something like:

Suggested change

	#[pyfunction]
	#[text_signature = "(graph, /, scale=1, center=None, resolution=0.35,
	equidistant=False)"]
	pub fn digraph_spiral_layout(
	graph: &digraph::PyDiGraph,
	scale: Option<f64>,
	center: Option<layout::Point>,
	resolution: Option<f64>,
	equidistant: Option<bool>,
	) -> Pos2DMapping {
	#[pyfunction(scale="1.0", resolution="0.35", equidistant="false")]
	#[text_signature = "(graph, /, scale=1, center=None, resolution=0.35,
	equidistant=False)"]
	pub fn digraph_spiral_layout(
	graph: &digraph::PyDiGraph,
	scale: f64,
	center: Option<layout::Point>,
	resolution: f64,
	equidistant: bool,
	) -> Pos2DMapping {

so we set the default in the macro generated python c api ffi function. But this also works fine