Make the following methods in vtkAxis virtual so users can customize vtkAxis:

GenerateSimpleLabel(), GenerateSprintfLabel(), GenerateLabelFormat(), GenerateLogSpacedLinearTicks()

This was originally proposed by a post on

http://vtk.1045678.n5.nabble.com/Request-Make-certain-methods-related-to-tick-labels-in-vtkAxis-virtual-td5744594.html

with the purpose of allowing the user to use customized axis labels, e.g. for displaying dates or times.

Apart from making the above methods virtual it is also required that the user can set the customized axis in a chart.
Hence further modifications were necessary in vtkChart, vtkChartXY and vtkChartXYZ to replace the axis created in the chart
by the customized axis.

Further a change was made to vtkAxis to allow the axis title to be switched off.