Many changes, mostly to _BaseVisualStim and GratingStim:
- ENH: new setter-getter interface using "stim.attribute = value" syntax. Done by using a AttributeSetter decorator.
- ENH: more checking of parameter correctness (val2array)
- ENH: renaming of variables to better reflect internal vs. user-relevant attributes.
- RF: reduction of code redundancy
- ENH: this lead to a performance improvement of ~40%