Additional NCFlex features and updates

[Fraser-Birks]

List of features added:

• A multi-processing parallel NCFlex script
• Functionality to build 3D kink-periodic fracture cells, including for special-case diamond 111 cells
• Ability to do continuation with kII, or both kI and kII keeping G constant
• Massively parallel script for kII continuation
• Massively parallel script for mode I convergence tests in NCFlex
• Ability to map the 2x1 Pandey surface reconstruction to a surface in any orientation
• Ability to map the 3x1 Silicon surface reconstruction to a surface
• Stable cluster atom sorting

[jameskermode]

Merging. Note that #203 will need to be revisited before too long.