[Describe your changes and what you've already done to test it.]

Check all that apply (change to [x])

• All changes meet libigl style-guidelines.
• Adds new .cpp file.
• Adds corresponding unit test.
• Adds corresponding python binding.
• This is a minor change.

Current implementation of cut_mesh function add duplicated vertices directly after the original one. Breaking the original indices of vertices is undesirable sometimes. This new function will append all new vertices at the bottom of V matrix and provides an index mapping to find the correspondence after cut.

Added some simple unit test. Since the mapping of indices changed, the test data for MIQ needs to be updated.