Fix colors for tetrahedral mesh

[stefsmeets]

Hi,

Tetrahedral meshes are converted to triangle meshes internally. If a color array is specified with the same length as the number of tetrahedra, meshplot gives an error: Invalid color array given! Supported are numpy arrays. <class 'numpy.ndarray'>

This PR adds a fix that repeats the color array 4 times, so that the color array matches the number of triangles generated.