Smoothed metallicity

Smoothed Metallicity is implemented for Gear. I still need to ensure that the code compile with none and EAGLE, but will do it once you accept the modifications.

You have a new example in SmoothedMetallicity that uses a glass with two different metallicities with some gaussian noise. At the discontinuity (+- the smoothing length), the transition is smoothed and gives the following graph

In this case, I am plotting in red the particles and in blue the theoretical solution (+- 1 sigma of the noise). The three plots corresponds to 3 different elements (I do not want to follow them all as it requires 9 graphs)

@aborissov, the code should be compatible with EAGLE, therefore you will just need to copy/paste