Add .NET 4.0 target (#335)

Consumers targeting 4.0 or 4.5 without the
.NET Core SDK will have to load the old 3.5 runtime
in order to run this library, which costs performance and
memory.
Adding this target allows those consumers to stay on the
4.0 runtime.

Author: angularsen

UnitsNet.Serialization.JsonNet/UnitsNet.Serialization.JsonNet.Signed.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="UnitsNet.Serialization.JsonNet.Common.props" />
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.0;net35</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;net35;net40</TargetFrameworks>
     <PackageId>UnitsNet.Serialization.JsonNet.Signed</PackageId>
     <Title>Units.NET Serialization with Json.NET (signed)</Title>
   </PropertyGroup>

UnitsNet.Serialization.JsonNet/UnitsNet.Serialization.JsonNet.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="UnitsNet.Serialization.JsonNet.Common.props" />
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.0;net35</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;net35;net40</TargetFrameworks>
   </PropertyGroup>
   <PropertyGroup>
     <!-- Workaround for building with dotnet CLI

UnitsNet/UnitsNet.NetStandard10.Signed.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="UnitsNet.Common.props" />
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.0;net35</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;net35;net40</TargetFrameworks>
     <PackageId>UnitsNet.Signed</PackageId>
     <Title>Units.NET (signed)</Title>
   </PropertyGroup>

UnitsNet/UnitsNet.NetStandard10.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="UnitsNet.Common.props" />
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.0;net35</TargetFrameworks>
+    <TargetFrameworks>netstandard1.0;net35;net40</TargetFrameworks>
     <RootNamespace>UnitsNet</RootNamespace>
   </PropertyGroup>
   <PropertyGroup>