Condition that describes the beam line

As described in Rec#288 and discussed at the end of last year, a better description of the interaction region should be available to reconstruction algorithms. The information should be:

• x, y, z of the average position,
• the covariance (spread) matrix,
• tx and ty describing the direction of the major axis.

The most straightforward place to obtain the numbers from is DeVP. The numbers should be in global coordinates, such that they can be directly used by reconstruction algorithms. The information should be stored in a condition that does not depend on alignment conditions.

Rec#288 will be used to discuss the source of information, a dedicated meeting on this topic will be organised.

I would suggest to add an interactionRegion method to DeVP that provides a numbers above in an appropriate struct. For backwards compatibility reasons, if the condition containing the information is not available the existing beamSpot method should be used to obtain x and y with reasonable default values for the covariance matrix and tx and ty set to 0.

An analogous method will be needed for DetDesc. It may be sufficient to have it return the backwards compatible information, as other values are not likely to be needed for simulation. To be confirmed.

FYI @mfontana, @lgrillo, @sborghi, @wouter, @dovombru, @mveghel