Use Google style docstrings

[mscroggs]

I suggest that we move to using Google style docstrings rather than Numpy style. The Google formatted strings are usually multiple lines shorter, and work more nicely with type hints.

Example Google docstring (proposed):

def element(family: str, cell: str, degree: int) -> FiniteElement:
    """"Create a finite element.

    Args:
        family: The element family
        cell: The cell
        degree: The polynomial degree
    """

Example Numpy docstring:

def element(family: str, cell: str, degree: int) -> FiniteElement:
    """"Create a finite element.

    Parameters
    -----------
    family: str
        The element family
    cell: str
        The cell
    degree: int
        The polynomial degree
    """