Add ability to pull a list value out in a pydm widget

[slacrherbst]

This add the ability to display a single list value in a pydm widget. Example channel value to achieve this is:

rogue://0/root.AxiVersion.TestListA/disp/3

There the last value is the index value.

There is a performance hit with this as the value has to be retrieved again to pull out the sliced value from the server:

def _updateVariable(self,path,varValue):

    if self._index != -1:
        varValue = self._node.getVariableValue(read=False, index=self._index)