attributeSetter work

Continuing the attributeSetter work. The goal is to make an attribute for every init parameter. This enables the subject to change parameters after initialization and to do it using operations. It also simplifies docstrings and the number of keywords that the subject has to remember. And setting/changing attributes becomes a bit faster as well.

With this commit I am approximately 50 % through the non-BaseVisualStim methods of the visual module.  The DotStim in particular has become much more flexible. Productive travel home from Nottingham!

All tests pass and I have consistently verified that stim.attribute = value produces the same output as stim.setAttribute(value) or stim = Stim(attribute=value) did before the edit.