latex variable with exponent causes compilation error

See: http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=3846

Quick fix is to require users to place brackets around latex variables containing exponents (and probably underscores)

Complete fix is to place braces around variables with lead/lags and those variables with no lead/lag but that are followed by an exponent. In the user's case, we would output: B^problem_t-1^\rho