AllPlot - Smoothing also for "Left Border"

Problem:
... data points of "Smoothing" settings are set to Zero in result data samples (first x data points)
... this cause e.g. Zero Altitude Values when Plot Smoothing is active
Solution:
... also most left values are using the same "smoothing" approach now (by using the data points left of them to create the average)

Tested - both in normal/compare mode and also after removing data points from the ride (using the GC Editor) - by removing at beginning, middle and end of ride file.

For more details check previous pull request - which was canceled since the patch did not work for the test cases listed above:

https://github.com/GoldenCheetah/GoldenCheetah/pull/1058