EnumerableRegistration for Excel Array formulae

Hi Govert
I ended up putting the Enumerable shim code into the Custom Registration project.
It might be a bit quirky to include in the master branch, but I'd be interested if you have any comments.

It automatically registers any function with signature IEnumerable&lt;T&gt; -> IEnumerable&lt;U&gt;, where T and U are classes or structs with at least one public property.
In Excel you invoke the function with an Array Formula.
The names of the public properties of the classes T and U are mapped to column headers in Excel.