implement is_valid_ref.vi input validation for all classes

[jimkring]

Currently, Table.lvclass and Item.lvclass implement input validation of object references.

Here is how is_valid_ref.vi raises an error if it's optional raise error if invalid? (False) input is set to True:

This should be used throughout the class to validate the reference prior to calling the DLL.

For example:

Notice how we use it in the constructor(s) and destructor, too, in addition to the various get/set VIs in the class.

Also, be sure to validate any additional object/reference arguments passed into a method, in addition to the primary reference, as shown below: