- add Calc.cum to aggregate Measurements.

- add ΣMeasurement type to represent result of this. It is very similar
  to Measurement, but every field comes accompanied with information
  about how much time there was no data for that field. In other words
  it is not all or nothing for NA in the result. For example a field
  might be present 90% of the time and NA only 10% of the time. We want to
  preserver knowledge about that 90% of valid values in the result. And we
  also want to know how much time there was no data.

- amend kpidemo.py and kpidemo.ipynb to demonstrate this.