Created by Julien Palard

The collatz sequence goes like this:

- Start by any number greater than zero.
- If your number is even, divide it by two.
- If your number os odd, multiply by three and add one.

For example, starting by `10`

we have:

```
10 → 5 → 16 → 8 → 4 → 2 → 1
```

Write a function, named `collatz_length`

, given a number the function return
the length of the sequence before reaching 1, for example:

```
>>> collatz_length(10)
6
```

Because it takes 6 steps (6 `→`

in the previous example) to reach 1.

Have fun.