refactored prob/internal_math so it works for higher order autodiff
fixed includes and internal_math calls for beta distr
removed camel case from gradRegIncGamma
fixed chi_square tests so they work with new internal_math
fixed gamma, gumbel, inv_chi_sq tests so they work with new internal_math
fixed inv_gamma, scaled_inv_chi_sq, student_t tests so they work with new internal_math