Function to get perimeter of a set of blocks/surfaces

[jamiebull1]

A function to get geomeppy to return the outline of all blocks would be useful for visualisation of results, and also for adding a single zone above an existing building made up of multiple zones (#99).

The call signature would be something like:

def perimeter(surfaces, level="ground"):
    """Returns the contiguous perimeter/s in x,y coordinates of a collection of surfaces.

    If there are non-contiguous groups they will be returned separately.

    :param surfaces: List of surfaces.
    :param level: One of "ground" or "roof". Defaults to "ground".
    :return: List of lists of coordinates for each contiguous group of surfaces.
    """