Document and check 'optimize' subroutine

[gvanuxem]

It could be good to document this hidden subroutine (exported though).

It's a generic routine that was not documented. It allows to easily use different algorithms on some problems with a generic interface.

For example :
($x, %ret) = optimize($x, %opt(f_func=>$func,
g_func=>$gfunc,
function='spg');

The routine adds usual default options if necessary (eps, gtol, ftol etc. depending on the routine used.).

See line 3869.

[mohawk2]

Pull requests welcome! :-)