Changed GlobalY in BoutMesh

o Added option mesh:symmetricGlobalY to follow symmetricGlobalX
  If enabled, this uses a definition of Y which is more useful
  for MMS testing.
MMS diffusion test in Y

o Fixed boundaries so they use Y in [0,2pi] like FieldFactory

o Using symmetricGlobalY setting, now get 2nd order convergence
Diffusion test in X,Y, and X-Y-Z

MMS/diffusion2 runs through three tests,
checking that each produces 2nd order convergence,
and creating a PDF figure of the error norm for each test.