pyxel V3.0

Publication of the ne version including DVC (analysis of 3D volume images).

pyxel v2.0

In addition to more comments and tutorials, the main modification concerns the way FE mesh are stored in the data structure.
The elements were stored as a python dict, where the key was the element number, whereas it is still a python dict but the keys are the element type and the items are homogeneous numpy array of node labels of all the elements of that type.