Bugfix in rplot

Make other plot types take into account that variables are in rows, not
columns
(cherry picked from commit e76828bb9cca0b4888b9f2d0ad3af568e4eeda92)
Add warning for variance decomposition at order=2

At order=2, the variance decomposition is still based on order=1. This
commit introduces a warning. See issue #278
(https://github.com/DynareTeam/dynare/issues/278). This information that
the variance decomposition is only triggered at periods=0 has been added
to the manual.
(cherry picked from commit 98ec0095dfe22acf3896c3dac74c4a71219fc019)