UniformSite inherits multiple wind speeds from Site

UniformSite is meant to be for a single wind speed, according to the docsting. But on line 299 of py_wake.site._site.py, it inherits the default_ws from Site, which is hard-coded as an array of values on line 18.

I propose that the single wind speed for UniformSite is specified at initiation. Perhaps like this:

class UniformSite(Site):
    def __init__(self, p_wd, ti, ws=12., interp_method='piecewise', alpha=None, h_ref=None):
        self.default_ws = ws

Is this solution sound ok, @mmpe @mikf @pire ? If it's fine, I'll make a feature branch and merge request.