fix typos in test_sim error messages
simplify TestObservable instantiation, add methods to manipulate attr

Another step toward Observable object for #17 and wrapping PA in data
space for #16.

For simplicity when creating an Observable object, make __init__ just
define default attributes and allow manipulation later. New method
setAttr allows changing many attributes in one line using keyword
arguments. May want to add some checking that it defines valid
attributes with reasonable ranges.

Also added wrapVector to defineDataVector method, which should allow a
likelihood code to properly account for the fact that some parameters
in the data vector may be wrapped.
add scratch/observable.py to flesh out new TestObservable class