Legacy TgcReadoutElement - Return pitches as double

Do not convert the strip pitched to pure integers when determining the center of the wire-group. Also it's been found that one wire needs to be subtracted to arrive at the actual centre of the group

Tagging: @pscholer, @kluit, @stavrop, @junpei, @masato

Tests have been performed on 165k data events filtered by the Zmumu stream. Apart from tiny improvements in the chi2/nDoF, all distributions are perfectly compatible DataChecks_TGC.pdf

Tagging: @jcatmore, @jchapman, @christos, @jdandoy for the offline-sw-review-approved label.