An object of class "unit_cell". It is a named list
of length 6 whose last three slots are of class
'angle'. Default cell parameters are a=10, b=20, c=15,
alpha=70, beta=80, gamma=100. When constrains due to
symmetry are required, b and c might be equaled to a,
alpha, beta and gamma might be set to 90, gamma might
be set to 120 and the three angles might be set
equal to each other.