MeshIO (input, output, convert) for (volumetric) meshes in .msh and .veg file format

New addition(s):

1. Added MSHMeshIO to read a mesh in .msh file format, and instantiate imstk::VolumetricMesh
2. Added a method in VegaMeshIO to convert imstk::VolumetricMesh to vega::VolumetricMesh
3. Added a method in VegaMeshIO to write imstk::Mesh in .veg file format.
4. Added an example testMshandVegaIO in Sandbox to test these implementations (Attached is the output with this merge request).