Enable (in)equality constraints for use with local and global scipy m…

…inimizers (#841)

* enable (in)equality constraints for use with scipy.optimize.minimize interface; support a few more minimisers; redefine _minimizer_callback signature to support trust-constr; add quick minimiser tests; minor touchups

* rename nested functions so they're not attempted to be tested individually

* fix scipy global solvers' configurations with or without constraints as much as possible

* add example global min. (DE) config file with theta23 inequality constraint

* add commented-out example DE scipy min. test configured from file

* superficial