Add a filter to compute 'distances' to the nearest characteristic GBs

This filter will calculate four numbers for each triangle. These numbers will be approximate distances to the nearest tilt, twist, symmetric, and 180 deg.-tilt boundaries. More details on these parameters can be found in:
- A.Morawiec, K.Glowinski, On "macroscopic" characterization of mixed grain boundaries, Acta Mater. 61, 5756-5767 (2013).
- K. Glowinski, On identification of symmetric and improperly quasi symmetric grain boundaries, J. Appl. Cryst. 47, 726–731 (2014).