Add option to specify radius steps

Allow user to specify the steps at which radius circles and labels will be drawn as, and also sets the rmax value to be a multiple of rstep. So for example a call to plot_windrose(t, r, kind='bar', normed=True, blowto=False, rstep=5) will generate radii/labels of 5,10,15 etc. I don't know if this the best way to achieve this (that is should this be part of WindroseAxes constructor or just passed around in kwargs?), it just worked for my use case.

windrose/windrose.py

@@ -82,6 +82,7 @@ def __init__(self, *args, **kwargs):
         PolarAxes.__init__(self, *args, **kwargs)
         self.set_aspect("equal", adjustable="box", anchor="C")
         self.radii_angle = 67.5
+        self.rstep = None
         self.cla()
 
     @staticmethod
@@ -137,8 +138,13 @@ def set_radii_angle(self, **kwargs):
         if angle is None:
             angle = self.radii_angle
         self.radii_angle = angle
-        N = 5
         rmax = self.get_rmax()
+        if self.rstep is not None:
+            rstep = self.rstep
+            N = np.int_(np.ceil(rmax/rstep))
+            rmax = N * rstep
+        else:
+            N = 5
         radii = np.linspace(0, rmax, N + 1)
         if rmax % N == 0:
             fmt = "%d"
@@ -346,7 +352,9 @@ def _init_plot(self, direction, var, **kwargs):
 
         normed = kwargs.pop("normed", False)
         blowto = kwargs.pop("blowto", False)
-
+        rstep = kwargs.pop("rstep", None)
+        self.rstep = rstep
+
         # Calm condition
         calm_limit = kwargs.pop("calm_limit", None)
         if calm_limit is not None: