Since pickle cannot handle pointer to member functions, this class wraps the call to
add minimum.
Parameters :
Ecut: float, optional
energy cutoff, don’t add minima which are higher in energy
max_n_minima : int, optional
keep only the max_n_minima with the lowest energies. If E is greater
than the minimum with the highest energy in the database, then don’t add
this minimum and return None. Else add this minimum and delete the minimum
with the highest energy. if max_n_minima < 0 then it is ignored.