Fix cargo clippy errors (#175)

In #174 the gnp random functions were extended to support values of 0
and 1 for the probability representing either empty of full graphs. In
that change comparisons between the value of probability and 1 were used
to check if it's a full graph and we should just fast path adding an
edge between every node. However, when running 'cargo clippy' on this it
rightfully points out that we should probably check for equality to 1
within an error margin (given that floating point is never exact).
Clippy suggested replacing using std::f64::EPSILON as the error value,
which is ~2.2e-16 [1] and replacing the comparison with:

(probability - 1.0).abs() < error

This commit just makes that change. While this is unlikely to cause any
issues in practice, because probability is a parameter and if someone is
going to want a full graph they'll likely call the function with
probability=1 in python. It's better to be safe just in case someone is
computing the probability value and could have some error on the value.

[1] https://doc.rust-lang.org/std/f64/constant.EPSILON.html

src/lib.rs

@@ -1479,7 +1479,7 @@ pub fn directed_gnp_random_graph(
         ));
     }
     if probability > 0.0 {
-        if probability == 1.0 {
+        if (probability - 1.0).abs() < std::f64::EPSILON {
             for u in 0..num_nodes {
                 for v in 0..num_nodes {
                     if u != v {
@@ -1585,7 +1585,7 @@ pub fn undirected_gnp_random_graph(
         ));
     }
     if probability > 0.0 {
-        if probability == 1.0 {
+        if (probability - 1.0).abs() < std::f64::EPSILON {
             for u in 0..num_nodes {
                 for v in u + 1..num_nodes {
                     let u_index = NodeIndex::new(u as usize);