Create element id input from multiple sources.

[kovacsv]

The solution is based on the approach of @SzamosiMate. The goal is to allow multiple inputs for every input that requires a list of element ids.

The valid inputs are:

• An ElementIdItemObj instance.
• An ElementIdObj instance.
• A string containing a guid.
• Any object with a Guid property.

Also added a utility function to get a list of element ids from an input without the need to care about casting.

ElementsObj inputElements = ElementsObj.Create (DA, 0);
if (inputElements == null) {
    AddRuntimeMessage (GH_RuntimeMessageLevel.Error, "Input ElementIds failed to collect data.");
    return;
}