Construct ΔQ from empirical data via linear interpolation

[HeinrichApfelmus]

The previous prototype included a function fromEmpirical to construct a ΔQ by linear interpolation of empirical data. This is missing from this version.

This is mostly a convenience function which can be implemented using existing functionality:

The function fromPositiveMeasure can be used to construct a ΔQ from a piecewise distribution function; the missing bit is the linear interpolation, which can be accomplished with lineFromTo from Numeric.Polynomial.Simple.

[semanticphilosopher]

My first issue is creating QTAs - typically these are stepwise and should be able to be created from appropriate dirac elements