Introduce the further constraint for the use of asymmetric unit

[qzhu2017]

For the generator, we previously considered the whole [0, 1] range.
It can be better handled in a smaller range based on the space group setting.

E.g. Pban has

Asymmetric unit
0 ≤ x ≤ 1/4 ; 0 ≤ y ≤ 1; 0 ≤ z ≤ 1/2