Data with 0 range generates warnings in reportGen

If a variable has a range of 0 (that is, all the same value), when we try to plot it min_value == max_value, which causes matplotlib to re-set the range to -0.1 through 0.1. More importantly, it emits useless warnings. We might as well set the range ourselves and eliminate the warnings.

$ glance reportgen --nolonlat --outputpath /tmp/eraseme output.nc output.nc 
/usr/lib/python2.7/dist-packages/matplotlib/axes/_base.py:2787: UserWarning: Attempting to set identical left==right results
in singular transformations; automatically expanding.
left=0.0, right=0.0
  'left=%s, right=%s') % (left, right))
/usr/lib/python2.7/dist-packages/matplotlib/axes/_base.py:3045: UserWarning: Attempting to set identical bottom==top results
in singular transformations; automatically expanding.
bottom=0.0, top=0.0
  'bottom=%s, top=%s') % (bottom, top))

Reference output.nc. It has one variable "zeros2d", which is full of zeroes.