Added a converter for tuple to values

[KRLango]

Added a converter from tuples to the value at a specified index in the tuple. This is for the Collection Index Section in nion-software/nionswift#1093

[cmeyer]

The TupleToValueConverter doesn't seem like a good general converter since it doesn't convert a value to another value - instead it accesses a property of a secondary object. I'll propose a different technique for this, but it might take a few days. In the interim, you can add these classes directly to Inspector.py so that they're isolated there.

Also the TupleOfIntegersToStringValueConverter is too specific to go into nionutils. If anything, we could introduce some type of CompositeConverter which would chain together a series of converters to make a single converter.