Update the timing of sTGC strip channel

This commit replaces the constant 25-ns time offset of sTGC strips by a look-up table. In a strip cluster, the middle strip has the same timing as the wire channel, and the adjacent strips has a time offset ranging from 10ns to 45ns depending on its position (in units of strip) relative to the middle strip.

Also many strip digits are created per energy deposited. This commit also creates strip digits without assuming a fixed number of strips per energy deposited as it is done previously. So the commit starts creating digit for the strip nearest the energy deposited and, then, loops over the neighbour strips until the charge integrated over the strip width is zero.