After the structure is centered, it is also aligned to a set of orthogonal axes.
This variable decides which set of axes to use. Only implemented for 3D, in which case
the default is inertia; otherwise none is default and the only legal value.
Options:
none:
Do not rotate. Will still give output regarding center of mass and moment of inertia.
inertia:
The axis of inertia.
pseudo_inertia:
Pseudo-axis of inertia, calculated considering all species to have equal mass.