BUG: TypeError when creating nose cones with bluffness between 0 and 1

**Describe the bug**

A "TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'int'" is raised when creating any nose cone with bluffness different of None or $0$.

**To Reproduce**

Create any nose cone with bluffness between $0$ and $1$. Minimal code to reproduce:

```
from rocketpy import NoseCone
NoseCone(length=1, kind="conical", base_radius=1, bluffness=0.5)
```
