implement numpy.mean

[Oceania2018]

Compute the arithmetic mean along the specified axis.
https://docs.scipy.org/doc/numpy-1.15.1/reference/generated/numpy.mean.html

[dotChris90]

ok - will do this - seems to be easy ;)

[dotChris90]

ok - pull request for first implementation of mean
keep issue open because API is not full implemented (missing multi dimensional stuff)

[Oceania2018]

Can you do NumPy.hstack first, other contributor is working on NumPy.mean.