Simple for loops and enumeration fails for NumericalData

[thomasms]

Simple loops fail in python such as:

import peakingduck as pd

data = pd.core.NumericalData([4,5,2,4.3,2.])

#this fails - why?
for x in data:
    print(x)

Get assertion failure:

Assertion failed: (index >= 0 && index < size()), function operator[], file /private/var/folders/tt/wpg8tb1x7yg3grvpdgv70n7m0000gn/T/pip-req-build-nsqrp4_g/thirdparty/eigen/Eigen/src/Core/DenseCoeffsBase.h, line 163.

This needs fixing urgently.