Add TriMeshEditor

[philippjfr]

This adds a class that makes it easy to edit very large TriMeshes by selecting just certain regions. Once a button is clicked the edits are then merged with the original mesh. Currently it supports:

• Moving mesh nodes
• Deleting mesh nodes

The following features should be supported:

• Marking regions to remesh, e.g. after deleting
• Ability to flip mesh edges by clicking on them
• Export a mesh where node indices are reset, i.e. consecutive integers (using a numba replace implementation)
• Support lasso-select for selecting submesh