Support for Excel Udfs inside Nested Classes

[Reported by Garrett Womack on the ExcelDna mailing-list](https://groups.google.com/forum/#!topic/exceldna/CmzfPSGG7iU).

>
> From: Garrett Womack
> Date: Thu, Apr 19, 2018 at 9:24 PM
> Subject: [ExcelDna] Excel Udfs in Nested Classes
> To: Excel-DNA
>
>
> Hi Govert,
>
> I noticed that Udfs aren't available in excel if you have them embedded in a nested class. Just wanted to point this out.
>
> ### === Works ===
>
> ```csharp
> public static class Udfs
> {
> // Put Udfs here
> }
> ```
>
> ### === Doesn't Work ===
>
> ```csharp
> public static class Udfs
> {
> public static class FinancialUdfs
> {
> // Put Udfs here
> }
> }
> ```
>