v2.2.3

This version features some small new features and small updates/bugfixes:

• added option to keep rest values of cross terms consistent with rest values of diagonal terms
• add option to remove cross terms when corresponding diagonal terms have negative force constants
• add option to specify the ei scales in qff-input-ei
• included control over convergence for the construction of perturbation trajectory
• several smaller fixes and updates in the code
• several fixed typos and updates in the documentation