In this merge request, I am implementing 2 things: the PopIII stars and the selection of elements from the parameter file.

The PopIII stars are the same than the other stars except that they use a different yields (and more) table. The selection of the type of stars is simply made on a metallicity threshold. As we are using 2 different tables that may not necessary share the same elements, I have implemented a selection of the elements from the yaml file.

I am also removing the possibility to read some parameters from the yaml file as the default value (from the table) can change between the first stars and the others.

Everything is inside GEAR's module, therefore it should be quick to merge.