RuntimeWarning with stats when stddev==0.0

glance stats and other paths that generate stats will generate a warning if the standard deviation for a variable is 0.0. (I believe only if the second file has a stddev of 0.0.) This is because the Pearson R calculation is not valid in that case.

The results are correct, but the warning is undesirable noise.

$ glance stats output.nc output.nc 
/usr/lib/python2.7/dist-packages/scipy/stats/stats.py:3003: RuntimeWarning: invalid value encountered in float_scalars
  r = r_num / r_den

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

In theory this can happen on any value (or even a set of values), but in practice it's tricky, since floating point rounding errors can cause the standard deviation to be larger than 0.0.