Support .NET 5 for EPPlus SDK and package as a .nupkg.

This change:
- Adds an SDK-style project for EPPlus, which is multi-targeted
- Drops support for the RIPEMD-160 hash, which is not supported in
  .NET 5. This is only used for (older) encrypted Excel files, which we
  don't support anyway.
- Adds Nuget packaging support for EPPlus

To generate the .nupkg, run:
dotnet pack EPPlus/EPPlusSDK.csproj -c Release

Change-Id: I7db0f3be1d7d28a891a61641a214a7d522ccc590

EPPlus/EPPlusSDK.csproj

diff --git a/EPPlus/EPPlusSDK.csproj b/EPPlus/EPPlusSDK.csproj
new file mode 100644
index 0000000..b2ec784
--- /dev/null
+++ b/EPPlus/EPPlusSDK.csproj
@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFrameworks>net472;net5.0</TargetFrameworks>
+    <RootNamespace>OfficeOpenXml</RootNamespace>
+    <AssemblyName>EPPlus</AssemblyName>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>OpenOfficeXml.snk</AssemblyOriginatorKeyFile>
+    <PackageId>Appsheet.EPPlus</PackageId>
+    <Version>1.0.0</Version>
+    <DefaultItemExcludes>$(DefaultItemExcludes);Properties/AssemblyInfo.cs;FormulaParsing/LexicalAnalysis/TokenSeparatorHandlers/**;FormulaParsing/LexicalAnalysis/TokenHandler.cs;FormulaParsing/Excel/Functions/Math/Rank.cs</DefaultItemExcludes>
+  </PropertyGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'net472'">
+    <Reference Include="System.Web" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
+    <PackageReference Include="System.CodeDom" Version="5.0.0" />
+    <PackageReference Include="System.Drawing.Common" Version="5.0.1" />
+    <PackageReference Include="System.Security.Cryptography.Pkcs" Version="5.0.1" />
+    <PackageReference Include="System.Security.Permissions" Version="5.0.0" />
+  </ItemGroup>
+</Project>

EPPlus/Encryption/EncryptionHandler.cs

diff --git a/EPPlus/Encryption/EncryptionHandler.cs b/EPPlus/Encryption/EncryptionHandler.cs
index 40bd3e7..11853da 100644
--- a/EPPlus/Encryption/EncryptionHandler.cs
+++ b/EPPlus/Encryption/EncryptionHandler.cs
@@ -279,8 +279,6 @@
         {
             switch (ei.HashAlgorithm)
             {
-                case eHashAlogorithm.RIPEMD160:
-                    return new HMACRIPEMD160(salt);
                 case eHashAlogorithm.MD5:
                     return new HMACMD5(salt);              
                 case eHashAlogorithm.SHA1:
@@ -629,8 +627,6 @@
             {
                 case eHashAlogorithm.MD5:
                         return new MD5CryptoServiceProvider();
-                case eHashAlogorithm.RIPEMD160:
-                        return new RIPEMD160Managed();
                 case eHashAlogorithm.SHA1:
                         return new SHA1CryptoServiceProvider();
                 case eHashAlogorithm.SHA256:

EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs

diff --git a/EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs b/EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs
index c839e14..aa4bcb1 100644
--- a/EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs
+++ b/EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs
@@ -22,10 +22,6 @@
  *******************************************************************************
  * Mats Alm   		                Added		                2013-12-03
  *******************************************************************************/
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
 using OfficeOpenXml.FormulaParsing.Excel.Functions.Database;
 using OfficeOpenXml.FormulaParsing.Excel.Functions.Text;
 using OfficeOpenXml.FormulaParsing.Excel.Functions.Math;

README.md

diff --git a/README.md b/README.md
index f79ab83..e2c1174 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,9 @@
 # EPPlus
 AppSheet fork of EPPlus
+
+To compile this fork, run:
+dotnet pack EPPlus/EPPlusSDK.csproj -c Release
+
+Then, copy the .nupkg file:
+EPPlus/bin/Release/Appsheet.EPPlus.1.0.0.nupkg -> jeenee/lib/nuget
+