fixed 3rd order deriv test for log1m, log1m_exp, log1p, log1p_exp
fixed 3rd order deriv tests for operators
fixed 3rd order deriv tests for abs, asin, asinh, acos, acosh
fixed 3rd order deriv tests for atan, atan2, atanh
fixed 3rd order deriv for functions

bessel first kind, bessel second kind, binary log loss, cart, ceil,
cos, cosh, digamma, ref, erfc, exp2, exp, expm1, fabs
fixed 3rd order deriv tests for all other functions

except gamma_p,gamma_q,binom_coef_log,Phi_approx,sign,
added higher order autodiff tests to binomial_coefficient_log
added higher order autodiff tests for gamma_p and gamma_q
added sign and phi_approx tests for higher order autodiff
fixed 3rd order autodiff tests for matrix functions:

columns_dot_product, columns_dot_self,dot_product, dot_self, mean, sd,
sum, variance, rows_dot_product, rows_dot_self
fixed 3rd order autodiff tests for:

cumulative sum, determinant, elt divide, elt multiply, exp, inverse,
log determinant, log, max, min, prod, softmax, trace
Merge branch 'develop' into feature/issue-675-check-autodiff
added more third order autodiff tests for:

crossprod, diag_matrix, diag_pre_multiply,diagonal, mdivide_left,
mdivide_left_tri_low, mdivide_right,
mdivide_right_tri_low,multiply_lower_tri_self_transpose_operator_additio
n,operator_division,operator_multiplication,operator_subtraction,tcrossp
rod
added fwd mode tests for sub_row
added fwd mode tests for sub_col
added fvar tests for head
added fvar tests for tail
added fvar tests for segment
fixed test naming for math/matrix head,segment,tail,subcol,subrow
added higher order tests for log_softmax