Remove 'mapping_3d' namespace. (#922)
It is removed from everywhere but 'scan_matching', 'pose_graph'
subfolders of 'mapping_3d'.
[Code structure RFC](e11bca586f/text/0000-code-structure.md)
It is removed from everywhere but 'scan_matching', 'pose_graph'
subfolders of 'mapping_2d'.
[Code structure RFC](e11bca586f/text/0000-code-structure.md)