Identify overlays by a name to be able to remove them

[bmatthieu3]

For overlays a name can be given in the options given in add_table, add_moc, add_stcs_footprints. It would be good to have a new method for removing them,

aladin.remove_overlay(name)

For this, aladin lite removeLayer function needs to be extended to search for an overlay by its name: https://github.com/cds-astro/aladin-lite/blob/develop/src/js/View.js#L1895