Add secondary spatial observable for generation of gain map of TRestDataSetGainMap

[AlvaroEzq]

Add the possibility to give a second observable to constrain the events that are selected as part of a segment.

This is useful, for example, for the case of a gain map of alphas, where you want to only take into account for the spectrum of each segments the alphas which tracks are fully contained in the segment. With this change, you can achieve that by using the origin and end observables as spatialObservable and spatialObservableSecondary