Unification of Element GeometryObject extraction

### Fix

There were a number of out of date methods for extracting Geometry from Revit Elements in `Element`.  I've removed the outdated ones.  Now there is only one - `InternalGeometry.`  Notably, this also fixes the issue encountered in #2487, as this method correctly translates instance geometry.  It also obviously eases maintainability as there is only one code path to maintain.

### Tweaks

#### AssemblyInfo.cs for RevitNodes

I've also tweaked the AssemblyInfo file for RevitNodes so it exposes it's internals to RevitNodesTests.  This allows me to remove the attribute from `InternalGeometry` and correctly set this method as `internal.`

#### Documentation

I've added new (trivial, but needed) documentation for these methods.  

### Tests

There is a new unit test for #2487 in RevitNodesTests.

### Reviewer

@Randy-Ma