RF/ENH: attributeSetters for RadialStim.
angularRes and visibleWedge can now be changed after initiation.
RF: moved clearTextures() to TextureMixin.
RF: moved mask/setMask to TextureMixin.
ENH: added texRes attribute in TextureMixin.
ENH: added maskParams to ElementArrayStim.
RF/ENH/ENH: attributeSetters for maskParams.
ENH: also for ElementArray and RadialStim.
ENH: use maskParams to set SD of gauss mask.
BF: accept operation=None in set*() functions.