* do not allocate xtol_abs unless needed

As discussed in #183, it is beneficial to avoid allocating potentially
huge buffers of size `n` unless `x`-tolerance criteria are used.

* bobyqa: handle xtol_abs == NULL

* cdirect: handle xtol_abs == NULL

* cdirect/hybrid: handle xtol_abs == NULL

* cobyla: handle xtol_abs == NULL

* sbplx: handle xtol_abs == NULL

* newuoa: handle xtol_abs == NULL

* praxis: handle xtol_abs == NULL

* optimize.c: handle xtol_abs == NULL

* implement generic algorithm paameters

* add test of generic params

* check return value of test

* more const info in param functions

* docs

* python docs

* python and MMA/CCSA param docs

* whoops

Collision was seen on NetBSD, ref:
http://mail-index.netbsd.org/tech-pkg/2020/10/11/msg023893.html

* avoid linker conflicts if we use nlopt-getopt.c with a libc that has getopt

* use nlopt-getopt.h if we are missing getopt or getopt.h

Closes #353

Fixes #349

Dembo paper was in 1983, not 1982, and I added the DOI URL too so it will be easier for others to find than it was for me.

Vector-valued constraints section listed `opt.add_inequality_mconstraint` twice, when it looks like it meant to list both of `opt.add_inequality_mconstraint` and `opt.add_equality_mconstraint`.

Closes #332

Closes #317

Closes #319

Closes #314

Visibility conditioned to specific cmake minimum version and compatible compiler id

By default, the mexFunctions seems to be hidden in the newer Matlab version, this fix force the compiler to keep the symbols visible.

Updating vintage 2010 data.

* Add usage of _MSVC_LANG for windows C++ standard check

* Use MSC_VER instead or WIN32

Detect wrong algorithm string

Also matlab's c++ library might be older and incompatible with the compiler's one,
so mention in the doc the possible workaround.

Added a test.

rm obsolete #ifdef NLOPT_UTIL_H in stogo code