added struct2array.m

fixes #58

struct2array.m

+function a = struct2array(s)
+%STRUCT2ARRAY Convert structure with doubles to an array.
+
+%   Author(s): R. Losada
+%   Copyright 1988-2013 The MathWorks, Inc.
+
+narginchk(1,1);
+
+% Convert structure to cell
+c = struct2cell(s);
+
+% Construct an array
+a = [c{:}];
+