correct doc for epsfcn in leastsq

See fdjac2 for determination of the step length. Original docs would lead you to beleave epsfcn is the actual step length
MAINT: io: Remove empty strings warning from idl.py

This warning has been in the code for quite a while (therefore people who
use it already know about it), and it can be rather annoying and verbose
when reading large arrays with empty strings because it is printed for each
occurrence. I propose to get rid of these warnings altogether.
MAINT: io: Remove multi-dimensional structures warning from idl.py

The warning has served its purpose, and is no longer necessary.

Also removed the filtering of the now nonexistent warnings from the
idl tests.
MAINT: io: remove a couple unused variables in idl.py (thanks, pyflakes)
Merge pull request #3498 from WarrenWeckesser/tiagopereira-patch-1

MAINT: io: remove old warnings from idl.py
Merge pull request #3495 from bmcage/docleastsq

correct doc for epsfcn in leastsq
ENH: initial commit of differential evolution code. A stochastic, population based, global optimizer
ENH: updated scipy/optimize/zeros.py to add differential_evolution
MAINT: changed name to _differentialevolution.py from _DEsolver.py
MAINT: refactoring _differentialevolution.py, specifically the DifferentialEvolutionSolver class
MAINT: refactoring _differentialevolution.py and test__differential_evolution.py
TST: added benchmarks for global optimizers, benchmark_global_optimizers.py.  Adding test functions for global optimizers to test_functions.py
DOC: adding to THANKS.txt, but also adding differential_evolution to  documentation: tutorial/optimize.rst
MAINT: refactoring _differentialevolution.py, test__differential_evolution.py, test_functions.py, benchmark_global_optimizers.py
TST: adding more test coverage to test__differential_evolution.py
ENH: added dithering to differential_evolution, made speed optimizations in _select_samples
ENH: refactoring strategies in _differentialevolution.py to remove boilerplate code. Improving documentation.
MAINT: refactoring mutate method in _differentialevolution.py
MAINT: changed the number of trials in benchmark_global_optimizers.py
MAINT: typos and BUG fix after refactoring _mutation (_differentialevolution.py), pep8 fixes to test_functions and test__differential_evolution