Is your feature request related to a problem?

When investigating the solid/liquid interface, I would like to take the outermost solid atoms as the reference and analyse the liquid close to the surface (e.g., <= 5 angstroms away from the surface in the direction perpendicular to the surface). However, the reference might shifts during the simulation, and I would like to find a selection updated on-the-fly.

Describe the solution you'd like

I'd like to add a new selection class RelPropertySelection, which can be used in the following way:
relprop [abs] property operator value selection
For example, if I want to choose the water with the z distances to the surface smaller than 3 ang, I can use:
relprop z < 3.0 surface
where surface is the selection of the outermost surface atoms.