ROBUSTLQ: The method F_to_K was returning the negative value of the value function matrix P.  Now returns P.
ROBUSTLQ: In the method robust_rule_simple, the if statement failed if you passed in an initial guess at P because it can't check 'if not [array]'.  Changed to if P_init is None which does work.
Merge branch 'tests' of github.com:jstac/quant-econ into tests