Added inheritance mechanism to intracellular class

[vincent-noel]

I started implementing a proper inheritance mechanism for PhysiBoSS, and I think it should become a part of the Intracellular class. The idea is to add a new method to the Intracellular interface :

void inherit(Cell* cell)

This will be called during the divide() function, to inherit the state of the mother cell in the daughter cell. Each intracellular implementation would have to implement it. I have a very early version for PhysiBoSS that Marco will test in the next days, but I already wanted to submit this to have your opinions.