Replace python mutable with its value in GetValue().

The "vtk.mutable" object is used to pass a python object to a C++
method that requires a reference argument.  Essentially, vtk.mutable
is a simple container for numeric types and strings, so that the C++
method can modify the contents and hence return an argument by
reference.  Somewhere during the wrapper update that occurred a few
years ago, the code that extracted the value from the vtk.mutable was
lost from the wrappers.  The vtk.mutable object still worked when it
was a container for numeric values (since it supports the number
protocol), but at that time it stopped working for strings.

Change-Id: I781a3ada1120143c9159968733997bf811fb6d90
Merge topic 'python-mutable-check' into master

ab5b27f9 Replace python mutable with its value in GetValue().