#5 Don't replace measured speed with estimated speed

[Ken]

In cases where the TCX input file already contains speed data from a sensor, the power should be added to the existing Extension:TPX node, rather than create a second estimated speed entry.

Example of output now:

<Extensions>
  <TPX CadenceSensor="Footpod">
    <Speed>5.577</Speed>
  <TPX>
<Extensions>
<Extensions>
<TPX xmlns="http://www.garmin.com/xmlschemas/ActivityExtension/v2">
<Speed>5.6</Speed>
<Watts>43.6431443539</Watts>
</TPX>