Avoid af.mean

[shyams2]

It was found that af.mean gives strange results when initial density of both species are unequal . For instance, when electrons are given a number density of 1, with ion density set to zero:

print('MEAN DENSITY')
n = nls.compute_moments('density')
print(af.mean(n[:, 0, :, :]))
print(af.mean(n[:, 1, :, :]))

print('MEAN DENSITY-2')
n = nls.compute_moments('density')
print(np.mean(np.array(n[:, 0, :, :])))
print(np.mean(np.array(n[:, 1, :, :])))

This gives the output:

MEAN DENSITY
0.5061728395061662
0.49382716049382086
MEAN DENSITY-2
0.9999999999999873
0.0

Similarly, with electron density = 1 and ion density = 2:

MEAN DENSITY
1.493827160493803
1.5061728395061484
MEAN DENSITY-2
0.9999999999999873
1.999999999999965

However, when both species are set with density = 1:

MEAN DENSITY
0.999999999999985
0.9999999999999848
MEAN DENSITY-2
0.9999999999999873
0.9999999999999825