test and documentation build errors fixed

- VariableResistor in src.Electrical.Analog.ideal_components modified
-- Unit = descriptor removed
-- No longer an UnboundError during Pkg.test()
- make.jl in docs modified
-- linkcheck_ignore added for the Mathworks link
-- It seems the link works and is valid when manually visiting in browser
-- No longer a 403 Forbidden error when running docs/make.jl

Author: matthew-kapp

docs/make.jl

@@ -31,6 +31,7 @@ makedocs(sitename = "ModelingToolkitStandardLibrary.jl",
         ModelingToolkitStandardLibrary.Hydraulic,
         ModelingToolkitStandardLibrary.Hydraulic.IsothermalCompressible],
     clean = true, doctest = false, linkcheck = true,
+    linkcheck_ignore = ["https://www.mathworks.com/help/simscape/ug/basic-principles-of-modeling-physical-networks.html#bq89sba-6"],
     warnonly = [:docs_block, :missing_docs, :cross_references],
     format = Documenter.HTML(assets = ["assets/favicon.ico"],
         canonical = "https://docs.sciml.ai/ModelingToolkitStandardLibrary/stable/"),

src/Electrical/Analog/ideal_components.jl

@@ -392,10 +392,9 @@ R = R_const + pos * R_ref * (1 + alpha * (port.T - T_ref))
 
     @parameters begin
         R_ref = 1.0,
-        [description = "Resistance at temperature T_ref when fully closed (pos=1.0)",
-            unit = "Ω"]
-        T_ref = 300.15, [description = "Reference temperature", unit = "K"]
-        R_const = 1e-3, [description = "Constant resistance between p and n", unit = "Ω"]
+        [description = "Resistance at temperature T_ref when fully closed (pos=1.0) (Ω)"]
+        T_ref = 300.15, [description = "Reference temperature (K)"]
+        R_const = 1e-3, [description = "Constant resistance between p and n (Ω)"]
     end
 
     @components begin
@@ -404,13 +403,12 @@ R = R_const + pos * R_ref * (1 + alpha * (port.T - T_ref))
 
     @variables begin
         pos(t), [description = "Position of the wiper (normally 0-1)"]
-        R(t), [description = "Resistance", unit = "Ω"]
+        R(t), [description = "Resistance (Ω)"]
     end
 
     if T_dep
         @parameters begin
-            alpha = 1e-3,
-            [description = "Temperature coefficient of resistance", unit = "K^-1"]
+            alpha = 1e-3, [description = "Temperature coefficient of resistance (K^-1)"]
         end
         @components begin
             port = HeatPort()