Improve start parameters of TRT segment track fit.

Improve start parameters of TRT segment track fits for tracks predominantly traversing the TRT endcaps which have an initial theta estimate close to zero, which may happen in rare cases.

The computation of the start parameters requires an estimate of theta which is now computed from the radial position of the first and last endcap measurements. If this still results in a theta close to zero, theta is estimated from the maximal radial distance between the inner or outermost radial extent of the first and last endcap TRT straw on track. Before the radial positions had been hard coded straw positions. The theta estimate also needs the z position of the first and last endcap straw, where before the z position of the innermost straw was incorrectly forced to zero.

Also detect endcap measurements using identifiers rather than z-coordinates.

This prevents a divergence of the global chi2 fitter which was leading to a floating point overflow.

Addresses ATLASRECTS-7262.