Chained shared evaluation

[rhattersley]

Compute the mean & standard deviation of the error with a single pass of the data.

>>> error = model - analysis
>>> isinstance(error, biggus.Array)
True
>>> mean_error = biggus.mean(error)
>>> std_error = biggus.std(error)
>>> mean_error, std_error = biggus.ndarrays([mean_error, std_error])

[rhattersley]

Done.