Matrix_exp and pre 2.13 Stan for RStan 2.13

Everything Ben needs for the next release of RStan has
been merged into stan-dev/math. We still want to update
to Boost 1.62 before we tag stan-dev/math for Stan 2.13.

The matrix_exp pull request hasn’t been merged into
stan-dev/stan yet:

Don’t try to merge it, just grab the one line you need
out of the function_signatures.h:

The relevant line of which is here:

add(“matrix_exp”, MATRIX_T, MATRIX_T);

  • Bob