strip_unapproved is an internal function which essentially 'deletes' layers from the current ternary plot in the event that
such layers are not one of the approved layers. Refer to approved_geometries for the current list of approved geometries.
Usage
strip_unapproved(layers)
Arguments
layers
list of the layers to strip unnaproved layers from.
Value
strip_unapproved returns a list of approved layers (may be empty if none are approved).