Effective viscosity bug (#133)

* Effective viscosity bug

Fixes the bug in `compute_elastoviscosity_εII` reported in #132

* Fix viscosity tests

Author: albert-de-montserrat

src/Viscosity/Viscosity.jl

@@ -178,6 +178,6 @@ end
         Base.@_inline_meta
         η = 0.0
         Base.@nexprs $N i -> !isplastic(v[i]) && (η += inv(fn(v[i], II, args)))
-        return η
+        return inv(η)
     end
-end
+end

test/test_Viscosity.jl

@@ -17,7 +17,7 @@ using Test, GeoParams
     @test η0 ==
         compute_viscosity_εII(rheology, εII, args) ==
         compute_viscosity_τII(rheology, τII, args)
-    @test 1(1 / η0 + 1 / el.G.val / dt) ==
+    @test 1/(1 / η0 + 1 / el.G.val / dt) ==
         compute_elastoviscosity_εII(rheology, εII, args) ==
         compute_elastoviscosity_τII(rheology, τII, args)
 
@@ -61,7 +61,7 @@ using Test, GeoParams
     @test η ==
         compute_viscosity_τII(rheology, τII, args) ==
         compute_viscosity_εII(rheology, εII, args)
-    @test 1(1 / η + 1 / el.G.val / dt) ==
+    @test 1/(1 / η + 1 / el.G.val / dt) ==
         compute_elastoviscosity_εII(rheology, εII, args) ==
         compute_elastoviscosity_τII(rheology, τII, args)
 end