Changed operation of physicsFeed in physics.c possibly breaking the examples in sbench (although they all still compile run)

[JamesTimothyMeech]

Change
The original code in physics.c would sum up all the sensor values in reading for all sensors except sensor 0.

I have modified the code such that the reading is the current value in time from the samples.txt for all sensors 0 to n.

Phillip expects that the original functionality is relied upon by the examples in sbench and therefore this change could break the examples. The output should be examined before and after this change to verify whether or not the output changes.