Fixed kernel multiplication with  numpy floats #2174

This PR fixes the multiplication of kernels with numpy floats. The bug was not easy to spot. The problem was that the `__rmul__` method of the kernel was not called, because the kernel has an `__array__` method implemented, which was used for the multiplication with numpy floats from the left. This behaviour is fixed now, by additionally implementing the `__array_wrap__` method, which checks for the context of the multiplication.  Furthermore I added some regression tests.
Oh, sorry to nitpick this, but it looks like you've inserted tabs.  Please just insert spaces such that it lines up the same way as other entries.