[MCH] improve track extrapolation to vtx wo MCS correction

[pillot]

When the track is extrapolated to the vertex (or MFT) without multiple scattering correction, the absorber material crossed (for e.g. E loss correction) is estimated by propagating the track from downstream, without information from upstream. This PR allows to provide an upstream point (typically the vertex or the MFT track position) to better estimate the path through the absorber. If no information is provided, the behavior is unchanged.

[github-actions]

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[lmassacr]

Hi @pillot, looks good to me. I just wonder if some changes need to be propagated accordingly in the DQ or QC framework, as I think the various extrapolation functions are used at several places. Since I saw you initialized the additional arguments in all functions to a default, I guess it might be fine, and those default values will always be picked whatever the extrapolation function was used before (even if with less arguments). But I just want to make sure with you.

[pillot]

Hi @lmassacr , yes, these changes should be transparent. The extra arguments are optional and if not provided (i.e. if zUpstream = nullopt, which is the default), as it is the case now everywhere these functions are used, the extrapolation is the same as before.
The idea would be to test if the MCH-MFT matching could be slightly improved by providing an upstream position in the extrapolation without Branson correction, then change the code in DQ accordingly.

[lmassacr]

Hi @pillot, ok thanks for the clarification. I am approving.