expose matrix_exp_multiply in Math to Stan

[yizhang-yiz]

Summary:

Allow Stan to use matrix_exp_multiply() and scale_matrix_exp_multiply().

Description:

The proposal is to overload matrix_exp interface so that in addition to
allow the following in Stan language

matrix_exp_multiply(matrix A, matrix B);
scale_matrix_exp_multiply(real t, matrix A, matrix B);

for exp(A)B and exp(At)B, respectively.

Reproducible Steps:

Function signature test.

Current Output:

n/a

Expected Output:

Pass function signature test

Additional Information:

Possible application: solve linear ODE systems.

Current Version:

v2.17.1

[bob-carpenter]

fixed with #2548