The RealData class stores the weightings as actual standard deviations and/or covariances.
Parameters :  x : array_like
y : array_like, optional
sx, sy : array_like, optional
sy : array_like, optional
covx : array_like, optional
covy : array_like, optional
fix : array_like
meta : dict


Notes
The weights needed for ODRPACK are generated onthefly with __getattr__ trickery.
sx and sy are converted to weights by dividing 1.0 by their squares. For example, wd = 1./numpy.power(`sx`, 2).
covx and covy are arrays of covariance matrices and are converted to weights by performing a matrix inversion on each observation’s covariance matrix. For example, we[i] = numpy.linalg.inv(covy[i]).
These arguments follow the same structured argument conventions as wd and we only restricted by their natures: sx and sy can’t be rank3, but covx and covy can be.
Only set either sx or covx (not both). Setting both will raise an exception. Same with sy and covy.
Methods
set_meta(**kwds)  Update the metadata dictionary with the keywords and data provided by keywords. 