Rename PeakProfile.yvalue --> PeakProfile.__call__.

Interface cleanup - let's use the PeakProfile class as a functor
whose instances can be directly called.
Fill docstrings for PDFEnvelope classes.
Fill in docstrings for PeakProfile class.
Add explicit peakprecision property to PeakProfile.
Improve module description, remove surplass imports.
Add wrapper for PeakProfile.ticker() method.
Fill docstring for PeakWidthModel.ticker().