Correcting delete on different datasets

NEW : new int number numberOfMeshes to help creating only new meshes
BUG : when opening a new dataset, and deleting everything the first
dataset was not deleted. Now we do not delete the old dataset from the
lists, and we just append the new files.
ENH : when adding new datasets, you only had the commons colorMaps
between those dataset. Now you can get back all the colorMaps from one
dataset if all the other datas are deleted : we compute the commons
colorMap each time a mesh is deleted