Add batchwise pure class

[church89]

This PR adds a new batchwise scheme class that allow to perform single material additions without running search_for_keff. A new method called add_material is added to the integrator class in abc.py that automatically initialized a BatchwisePure class.

A public abstract method get_root function has also replaced the private _get_cell_attrib() returning different values according to the batchwise scheme used.