Types of subsets:
  X View: all points from a curve, within a specified X range
  Y View: all points from a curve, within a specified X range
  XY View: all points from a curve, within a specified X and Y range
  Flaged: all points which have not been flaged by a flag vector.

Example situations:
  Fitting to a data subset
  statistics scalars on a data subset
  FFT type anaysis on a data subset

Update requirements:
  May want to keep locked to current plot view
  May want to stay fixed once set.

So: 
1) create a KstView which 
  inputs: Vx, Vy, Uneven Vector interpolation rules
  outputs: two vectors, Vxout, Vyout
KstView is inherited by KstXView which takes
    Sxmin, Sxmax
KstView is inherited by KstYView which takes
    Symin, Symax
KstView is inherited by KstXYView which takes
    Sxmin, Sxmax, Symin, Symax
KstView is inherited by KstFlagView which takes
    Vflag

2) let plots output scalars (xmin, xmax, ymin, ymax)

With these additions, given the correct UI, we can create any object
that we might need.

UI:
A general NewView Dialog which can create any of the above views.
Appropriate options in the fit dialog to create one of the above views.
rmb options in 2dPlots which create X, Y, or XY views with sensible options.
The dialogs need easy access to the Plot Range scalars (and should default
appropriately).


