Fix reading errors as an array

[billetd]

As it stands, passing an array to the error keyword in the Data class doesn't work, as checks if error == 0:. This fails if error is an array of the same length of values.

A simple fix for this is to check if error has a size of 1, which is also equal to zero. The size check is actually done later at L187 of data.py, but the code will fail before getting to that point.

[billetd]

Just realised this doesn't work if you give a single value for error. Fixing now.

[klaundal]

Thanks! Weird that we had not encountered that before. Must have used scalar values for the error every time.