Extend system descriptions

Author: hersle

src/systems/abstractsystem.jl

@@ -2972,12 +2972,13 @@ function extend(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nam
     cevs = union(get_continuous_events(basesys), get_continuous_events(sys))
     devs = union(get_discrete_events(basesys), get_discrete_events(sys))
     defs = merge(get_defaults(basesys), get_defaults(sys)) # prefer `sys`
+    desc = join(filter(desc -> !isempty(desc), get_description.([sys, basesys])), " ") # concatenate non-empty descriptions with space
     meta = union_nothing(get_metadata(basesys), get_metadata(sys))
     syss = union(get_systems(basesys), get_systems(sys))
     args = length(ivs) == 0 ? (eqs, sts, ps) : (eqs, ivs[1], sts, ps)
     kwargs = (parameter_dependencies = dep_ps, observed = obs, continuous_events = cevs,
         discrete_events = devs, defaults = defs, systems = syss, metadata = meta,
-        name = name, gui_metadata = gui_metadata)
+        name = name, description = desc, gui_metadata = gui_metadata)
 
     # collect fields specific to some system types
     if basesys isa ODESystem