GDI/{Single,Paint}Window: use HOLLOW_BRUSH for hbrBackground

hbrBackground==nullptr means that WM_ERASEBKGND is used, see:

 http://blogs.msdn.com/b/oldnewthing/archive/2014/03/05/10505179.aspx

This is a micro-optimization to save this message.
BarographRenderer: eliminate redundant "no data" check

We already check if the LeastSquares object has a valid result.  This
special case can never happen.
WindChartRenderer: simplify zero division workaround
WindChartRenderer: remove unnecessary "fixed" casts
Math/LeastSquares: clear all slots in Reset()
Math/LeastSquares: no implicit initialisation

Reduce bloat.
Math/LeastSquares: use range-based for
Math/LeastSquares: make all attributes private

Add public accessor methods.
Math/LeastSquares: move API documentation to header
Math/LeastSquares: shorter method names

Remove the "LeastSquares" prefix from method names; we're already in
the LeastSquares class.
Math/LeastSquares: use GetYAt() internally