Add basic user contributed Cython function as an example.

Author: rpep

fidimag/user/example.pyx

@@ -0,0 +1,12 @@
+from libc.math cimport sin, M_PI
+import numpy as np 
+cimport numpy as np
+
+
+
+def fast_sin_init(mesh, double[:] field, *params):
+    t, axis, Bmax, fc = params
+    for i in range(mesh.n):
+        field[3*i+0] = Bmax * axis[0] * sin(fc*t)
+        field[3*i+1] = Bmax * axis[1] * sin(fc*t)
+        field[3*i+2] = Bmax * axis[2] * sin(fc*t)