Fix setting of default method and starting location

* Accept functions that implement only F(), FDf() or F(), DF() methods
* Update function call stats also when setting the starting location
Merge branch 'fix-starting-location' of https://github.com/vladimir-ch/gonum-optimize into vladimir-ch-fix-starting-location

Conflicts:
	local.go
Merge branch 'vladimir-ch-fix-starting-location'