Disallow setting of core construct definition properties when idref is set

[bworrell]

If an idref is set on an Indicator or other core construct, we unset the id_ property. It might be helpful to disallow the setting and exporting of other instance properties when idref is set, such as xsi:type. This would help address some of the concerns in #172

[imjonsnooow]

Question: should we go with a 'set it and forget it' method, or do we want to actively prohibit setting properties when idref is set? The easiest solution would be to check if idref is set in API objects' to_obj method, and not convert properties if idref is set. However, if we want to throw errors in these conditions, we'll have to modify all of the setters for all of the objects.

Any preference as to which route we should go? I'm guessing we'll want to go with the latter, so people aren't confused when they try to set properties and those properties are silently dropped...