Implement 2D measurements

Implementation of 2D measurements (for VP), rather than performing 2x 1D projection and filter steps for measurements that are actually of the form (X,Y). Saves filter/projection/minimise calls, extrapolation calls and aids in outlier removal.

Measurement type VP2D is added, logic is redone for N-dimensions, and a std::variant member of FitNode takes care of all dimension-dependent properties and calls.

The corresponding math is implemented in the LHCb math kernel in this MR: LHCb!2660 (merged)

2D measurements can be turned on by setting VPMeasurementProvider::Use2D = True as is done in the test here: Moore!983 (merged)

Connected downstream MRs: Moore!983 (merged), Alignment!208 (merged), Panoramix!115 (merged)

Reference updates: Moore!1340 (merged) Alignment!234 (merged) Brunel!1198 (closed)

Wishlist:

• understand reason for minus sign in projectionmatrix for Y.
• investigate 2D measurements for Muons as well (not just VP)
• alignmentDerivatives for 2D measurements (in TrackTools/MeasurementProviderProjector)

Follow-up issue here: #211

mentioned in merge request LHCb!2660 (merged)

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mentioned in merge request !2213 (merged)

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• e0ccaa73 - tested 2D measurements, corrected many 2D filter aspects in Similarity....

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• efa6becc - used LHCb::doubleVariant for shorter notation
• 2ad66114 - implemented some of Gerhards suggestions for cleaner code
• 6554aa7b - resolved Gerhards suggestions - no more variants in Measurement itself, calls...

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