Lusas_Toolkit: Detect HOFs when deleting elements

When deleting points, the method should recognise when a point is part of a higher order feature (such as a line or surface) and return a warning.

We could probably do this in a single Linq query and group in to one warning.

e.g. "P1, P2, P3 and P4" define higher order features and therefore cannot be deleted.

The same would go for lines, and it will not be too difficult for surfaces (if solids are implemented).

Locate the element-deletion method and existing warning handling, then inspect how points are referenced by lines and surfaces. Confirm how deletion is currently performed before deciding where the higher-order-feature check belongs. Done means deleting referenced points returns one grouped warning naming them, with equivalent handling for lines and surfaces.

Written by the indexing model from the issue text.