Generalize to allow FD approximation using boost multiprecision number types

Currently, the only use of FD is to double check the derivatives computed
either by hand-written code or (more frequently) by automatic differentiation.
However, only multiprecision number types allow to really be sure that the
rounding/cut off errors of FD do not hide subtle errors.

Unfortunately, this patch doesn't go all the way.  It now won't run with 'double'
anymore.  Also, user code needs to be modified in small but inelegant ways
(e.g., no 'std::' prefixes for math functions).  Since the FD tests don't
have to be run every day that's okay for now.

[[Imported from SVN: r9884]]