Create extension methods for Facet field types

[NightOwl888]

To make adding fields more discoverable via Intellisense, we have already created extension methods for "normal" field types here. However, we still need extension methods for facet field types defined within Lucene.Net.Facet to make the API consistent.

Proposed API

namespace Lucene.Net.Documents.Extensions
{
    public static class DocumentExtensions // Check to ensure the name doesn't collide with that in the Lucene.Net assembly, rename to DocumentFacetExtensions if necessary
    {
        public static FacetField AddFacetField(this document, string dim, params string[] path);
        public static SortedSetDocValuesFacetField AddSortedSetDocValuesFacetField(this document, string dim, string label);
        public static AssociationFacetField AddAssociationFacetField(this document, BytesRef assoc, string dim, params string[] path);
        public static Int32AssociationFacetField AddInt32AssociationFacetField(this document, int assoc, string dim, params string[] path);
        public static SingleAssociationFacetField AddSingleAssociationFacetField(this document, float assoc, string dim, params string[] path);
    }
}

Unit Tests

In addition, there are currently no tests for these extension methods, as the tests are using the existing Java API syntax. We should set up some basic unit tests to ensure the correct field type is added to a Document with the correct values and returned from the extension method correctly (both for the original extension methods and for the new facet extension methods).

[NightOwl888]

It turns out these extension methods already existed, but they were in in the Lucene.Net.Documents namespace so I missed them in Intellisense. So, #468 moves them to the correct namespace Lucene.Net.Documents.Extensions and adds tests to close this issue.

[rclabo]

Just bumped into this breaking change but it makes perfect sense these were moved into an Lucene.Net.Documents.Extension.DocumentExtensions class. And thankfully in Visual Studio, ctl + . makes it so easy to find the new namespace that needs added to a project to access these extension methods.