* User interface

  The following improvements shall ensure similiar GUI features as in
  old Win3.x version.

  - Function: cairoPlot2d
    Make axisX.start and axisX.stop positions dependent of precision

  - Function: createAxisLabel
    Parametrize margin by precision

  - Function: filterResponseTimeNew
    Allow user break

  - Feature: Response Plot
    Display coordinate when mouse moves over graph area

  - Feature: Help
    Implement Yelp based help system


* Functionality

  - File: dfcProject
    Introduce an option to export in second-order sections form, as
    described in MathWorks Help Center for function tf2sos().
    https://www.mathworks.com/help/signal/ref/tf2sos.html

  - Function: responseWinCreate
    Restore size of drawing area from last session

  - Function: dfcPrjExport
    Create and use an export precision (for coefficients) instead of using
    the output precision

  - Function: plotPhase
    Check phase response of linear FIR filters and improve the graph layout


* Maintainability

  - File: mathMisc.c
    Implement a generic math. error handler

  - Function: mathPolyCoeffs2Roots
    Try to avoid mixing error codes from gsl_errno.h and errno.h 

  - Function: ftrBandstop
    Handle GSL_POSINF for call to filterResponsePoly() 
