Add method for loading a color layer from a textured mesh

[Jaeyoung-Lim]

Problem Description
This PR was originally proposed in ANYbotics#374, but being backported to this fork due to the upstream PR not being reviewed

When loading a gridmap from a polygon mesh, only the geometry was being read on the grid map.

This commit extends the gird map pcl coverter for loading a color layer from a textured mesh

This is useful when loading colored pointclouds or reconstructions on the grid map for visualization purposes

Testing
A mesh can be read and added with a color layer with the two sequences

  grid_map::GridMapPclConverter::addLayerFromPolygonMesh(mesh, "elevation", grid_map_);
  grid_map::GridMapPclConverter::addColorLayerFromPolygonMesh(mesh, "color", grid_map_);