Changed varargin check to isempty() for speed

The previous version used length(varargin) rather than isempty(varargin)
to determine if inputs were given.

isempty method should be faster
Merge branch 'master' of https://github.com/qdev-dk/matlab-qd
Merge branch 'master' of https://github.com/qdev-dk/matlab-qd
Merge branch 'master' of https://github.com/qdev-dk/matlab-qd