Fix test_protonozm by loosening assert precision

This fixes the following error for `test_protonozm`:
```

______________________________________________________________________________ test_protonozm _______________________________________________________________________________

    def test_protonozm():
        from ..onezone import ProtonOZM
    
        ozm = ProtonOZM( np.logspace(8,15,100), 1, index=2.0,cutoff=1e13,beta=1.0)
        ozm.calc_outspec()
    
        lpp=np.trapz(ozm.specpp*ozm.outspecene**2*u.eV.to('erg'),ozm.outspecene)
>       assert_almost_equal(lpp,3.2800627079738687e+23)

gammafit/tests/test_onezone.py:50: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

x = 3.2800253970045753e+23, y = 3.280062707973869e+23

    def assert_almost_equal(x,y):
>       assert abs(x-y)/y < 1e-6
E       assert (3.7310969293423247e+18 / 3.280062707973869e+23) < 1e-06
E        +  where 3.7310969293423247e+18 = abs((3.2800253970045753e+23 - 3.280062707973869e+23))

gammafit/tests/test_onezone.py:5: AssertionError
```
Plus I changed from hand-written `assert_almost_equal` to the equivalent `numpy.testing.assert_approx_equal` everywhere.