Add SFM Visualiser

• VTK-based visualiser class Code/Lib/sfmVisualiser
• Example app to show useage `Code/CommandLineApps/sfmVisualiserDemo.cpp
• Requires VTK developer library installed, see README.md for

Bug fixes and minor improvements

• Ignore periodic boundary conditions when calculating difference between two pos2d's
• Make vec2d, pos2d and dir2d operator overloads reference arguments const so they work with r-values. You can now perform the following types of operations:

sfm::dir2d difference = sfm::pos2d(2.0, 1.0) - sfm::pos2d(1.0, 2.0)

• Add operator for scaling dir2d's by a float
• Add default constructors that initialise pos2d and dir2d to (0,0)