sagemathgh-37218: Fix to ncsym.ncsym.nesting

    
The nesting function in combinat.ncsym.ncsym.py does not operate as
described in its documentation.  As intended, it should compare all arcs
of two set partitions and count the number of times the second arc nests
in the first.  As written, there was a break statement caused many
(necessary) comparisons to be skipped, particularly for set partitions
with large blocks.

I have re-written the function so that all necessary comparisons are
made.  I have also added an example in the docstring that was computed
incorrectly by the old code, but is corrected by my fix.

This bug is not a previously known issue, and I have also confirmed with
the original author (@tscrim ) that no one else is currently working on
a fix.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.

### ⌛ Dependencies

No dependencies
    
URL: sagemath#37218
Reported by: lucasgagnon
Reviewer(s): Travis Scrimshaw

src/sage/combinat/ncsym/ncsym.py

@@ -103,6 +103,13 @@ def nesting(la, nu):
         sage: nesting(set(mu).difference(nu), nu)
         1
 
+        sage: A = SetPartition([[1], [2,5], [3,4]])
+        sage: B = SetPartition([[1,3,4], [2,5]])
+        sage: nesting(A, B)
+        1
+        sage: nesting(B, A)
+        1
+        
     ::
 
         sage: lst = list(SetPartitions(4))
@@ -135,11 +142,13 @@ def nesting(la, nu):
     nst = 0
     for p in la:
         p = sorted(p)
-        for i in range(len(p)-1):
-            for a in arcs:
-                if a[0] >= p[i]:
+        for a in arcs:
+            if p[-1] < a[0]:
+                continue
+            for i in range(len(p)-1):
+                if a[1] <= p[i+1]:
                     break
-                if p[i+1] < a[1]:
+                if a[0] < p[i]:
                     nst += 1
     return nst