Uniform dimensions which are symmetric about zero can now be non-locally access with the negative of the dimension name.

For example, if you have a dimension 'x' with domain (-10, 10) and it uses one of the dft/dct/dst transforms then you can access quantities non-locally using '-x', as in phi(x => -x).

Updated documentation to this effect and added a testcase.

git-svn-id: svn+ssh://svn.code.sf.net/p/xmds/code/trunk/xpdeint@2857 41b19578-b77a-4e3b-9682-c8570d237623