gh-40887: convert some methods in designs and graphs to n_*
    
This replaces methods `num_points` and `num_blocks` by `n_points` and
`n_blocks` in designs.

Old names are kept as aliases.

cf also #40875

This is just a small reduction of the number of method with names
`num_*`

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
    
URL: #40887
Reported by: Frédéric Chapoton
Reviewer(s): Vincent Macri

Author: Release Manager

src/sage/combinat/designs/bibd.py

@@ -334,7 +334,7 @@ def balanced_incomplete_block_design(v, k, lambd=1, existence=False, use_LJCR=Fa
                     return False
                 raise EmptySetError(f"there exists no ({v},{k},{lambd})-BIBD")
             B = B.incidence_structure()
-            if B.num_blocks() == expected_n_of_blocks:
+            if B.n_blocks() == expected_n_of_blocks:
                 if existence:
                     return True
                 else:
@@ -1467,7 +1467,7 @@ def __repr__(self):
             (13,3,1)-Balanced Incomplete Block Design
         """
         bsizes = list(frozenset(self.block_sizes()))
-        return "Pairwise Balanced Design on {} points with sets of sizes in {}".format(self.num_points(), bsizes)
+        return "Pairwise Balanced Design on {} points with sets of sizes in {}".format(self.n_points(), bsizes)
 
 
 class BalancedIncompleteBlockDesign(PairwiseBalancedDesign):
@@ -1526,10 +1526,10 @@ def __repr__(self):
             sage: b=designs.balanced_incomplete_block_design(9,3); b
             (9,3,1)-Balanced Incomplete Block Design
         """
-        v = self.num_points()
+        v = self.n_points()
         k = len(self._blocks[0]) if self._blocks else 0
         l = self._lambd
-        return "({},{},{})-Balanced Incomplete Block Design".format(v,k,l)
+        return f"({v},{k},{l})-Balanced Incomplete Block Design"
 
     def arc(self, s=2, solver=None, verbose=0, *, integrality_tolerance=1e-3):
         r"""

src/sage/combinat/designs/gen_quadrangles_with_spread.pyx

@@ -217,7 +217,7 @@ def dual_GQ_ovoid(GQ, O):
 
     # GQ.ground_set()[i] becomes newBlocks[i]
     # GQ.blocks()[i] becomes i
-    newBlocks = [[] for _ in range(GQ.num_points())]
+    newBlocks = [[] for _ in range(GQ.n_points())]
     pointsToInt = {p: i for i, p in enumerate(GQ.ground_set())}
 
     for i, b in enumerate(GQ.blocks()):

src/sage/combinat/designs/group_divisible_designs.py

@@ -292,7 +292,7 @@ def __init__(self, points, groups, blocks, G=None, K=None, lambd=1,
 
         if check or groups is None:
             is_gdd = is_group_divisible_design(self._groups, self._blocks,
-                                               self.num_points(), G, K,
+                                               self.n_points(), G, K,
                                                lambd, verbose=1)
             assert is_gdd
             if groups is None:
@@ -355,6 +355,6 @@ def __repr__(self):
         if not gdd_type:
             gdd_type = "1^0"
 
-        v = self.num_points()
+        v = self.n_points()
 
         return "Group Divisible Design on {} points of type {}".format(v, gdd_type)