Implementing widgets

[x] Mesh3D has now loadMesh function
[  ] LoadMesh function might be needed to modified further in order that the colors are handled more appropriately
[x] Implemented Mesh Widget
* Constructor is receiving Mesh3D 
* Polygons(faces) are now represented by one Mat instead of std::vector<std::vector>>
* Polygons matrix have to be 1 dimensional (as if it were a std::vector). 
* Each face is preceded by the number of vertices within the face, and represented by the indices of the corresponding vertices, such as 3 10 12 15 = 3 indices that are at the indices 10, 12, 15
* NaN masking is also implemented as in CloudWidget, however, since the vertices in the cloud are referred to by faces, a LUT (look up table) is constructed.
[x] Moved vtkpoint_data back to viz.cpp
* To avoid compiling errors
* Removed static and inline keywords, and added vtkpoint_data declarations to the beginning of the cloud_widgets and shape_widgets
[  ] Need to solve the compiling problem related to vtkpoints_data in a more elegant way