Add new util function getSlicedProfiles() to average quantities over specified subdomains.

This new feature allows the user to average solid volume fraction and velocity over subdomains of the box. This is useful to get the profiles of the quantities only over specific zones of the domain rather than over the whole domain.

The algorithm is quite the same as the one from getDepthProfiles() function. The major change is that the volume fraction is computed considering that the volume of the spheres can be sliced in two different directions instead of only one in the profile direction.

This function also allows to define a discontinuous subdomain made of different slices. This can be useful to exclude specific zones from the averaging procedure or to target similar zones like symmetric boundaries.

A more detailed description of the function is given in the doc in py/_utils.cpp

The function has been tested and give the same results as the one from getDepthProfiles when averaging over the whole domain.