Debug Weisfeiler_lehman

gmatch4py/kernels/weisfeiler_lehman.pyx

@@ -142,4 +142,4 @@ cdef class WeisfeleirLehmanKernel(Base):
             k += np.dot(phi.transpose(), phi)
             it = it + 1
 
-        return minmax_scale(k).todense()
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+        return np.ma.getdata(minmax_scale(k))
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