Quickff

Update the handling of how to locate data files

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

This release is described in the following published paper:

Extension of the QuickFF force field protocol for an improved accuracy of structural, vibrational, mechanical and thermal properties of Metal Organic Frameworks, L. Vanduyfhuys, S. Vandenbrande, J. Wieme, M. Waroquier, T. Verstraelen, V. Van Speybroeck, Journal of Computational Chemistry
http://dx.doi.org/10.1002/jcc.25173

Includes better description of bend terms with rest values of 180 degrees. Also contains some bugfixes and cleaning.