add citations/literature to numbers of accuracy/drift in introduction

[soerenthomsen]

@ptestor commented:

"Concerning the whole section 2 (Introduction), I have the feeling that we would need more references (even grey literature) when numbers are provided, about accuracy/drift for instance."

[soerenthomsen]

Good point. We have one open issue related to the accuracy of the multipoint calibration #48 (a key issue!)

In my view it is key to clearly distinguish between manufacture values and experiences from real users.

We mention and cite some real examples of drift in the dmqc section.

@tomhull maybe some of them we can cite here to when appropriate?