sagemathgh-37956: faster implementation of method `sage.graphs.generators.families.BalancedTree`
    
We propose a direct implementation of the graph generator `BalancedTree`
that is faster than the previous method using networkx.

Before (using networkx)
```
sage: %timeit G = graphs.BalancedTree(3, 5)
550 µs ± 1.17 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
sage: %timeit G = graphs.BalancedTree(3, 7)
5.33 ms ± 37.5 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
sage: %timeit G = graphs.BalancedTree(4, 5)
2.13 ms ± 18.4 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
sage: %timeit G = graphs.BalancedTree(4, 7)
54.5 ms ± 278 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```

With this PR
```
sage: %timeit G = graphs.BalancedTree(3, 5)
157 µs ± 809 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)
sage: %timeit G = graphs.BalancedTree(3, 7)
1.44 ms ± 4.94 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops
each)
sage: %timeit G = graphs.BalancedTree(4, 5)
569 µs ± 6.78 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
sage: %timeit G = graphs.BalancedTree(4, 7)
9.52 ms ± 61.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
```


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

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

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37956
Reported by: David Coudert
Reviewer(s): David Coudert, Travis Scrimshaw

sagemathgh-37956: faster implementation of method `sage.graphs.generators.families.BalancedTree`
    
We propose a direct implementation of the graph generator `BalancedTree`
that is faster than the previous method using networkx.

Before (using networkx)
```
sage: %timeit G = graphs.BalancedTree(3, 5)
550 µs ± 1.17 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
sage: %timeit G = graphs.BalancedTree(3, 7)
5.33 ms ± 37.5 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
sage: %timeit G = graphs.BalancedTree(4, 5)
2.13 ms ± 18.4 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
sage: %timeit G = graphs.BalancedTree(4, 7)
54.5 ms ± 278 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```

With this PR
```
sage: %timeit G = graphs.BalancedTree(3, 5)
157 µs ± 809 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)
sage: %timeit G = graphs.BalancedTree(3, 7)
1.44 ms ± 4.94 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops
each)
sage: %timeit G = graphs.BalancedTree(4, 5)
569 µs ± 6.78 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
sage: %timeit G = graphs.BalancedTree(4, 7)
9.52 ms ± 61.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
```


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

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

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37956
Reported by: David Coudert
Reviewer(s): David Coudert, Travis Scrimshaw