Add figure title for rplot
Clarify theoretical second moments for order=2

Eliminates warning message introduced in
4c8f3a89cc91ef301cf2a20d68d7dfb0cb983254
Adds hint to approximation in table title and adds the information with
a reference to Kim/Kim/Schaumburg/Sims (2008) to manual.
closes #278
Restrict endogenous prior to stationary models and allow for measurement error