support selections on contiguous containers (e.g. vector<v2 Track>...)

See task 3 in these slides.

• Add class Pr::Selection<T> to support selections on contiguous storage containers (i.e. LHCb::span<T>). This is implemented as span + vector of indices.
• Add tests for Pr::Selection<T>.

Add two algorithms:

• One (Pr::Filter<T>) that applies a selection cut to Pr::Selection<T> input, producing another Pr::Selection<T>. The selection cut is specified via LoKi. This MR depends on !1285 (merged) so that LoKi cuts can be applied to v2::Track objects.
• One (Pr::MakeSelection<T>) that creates a Pr::Selection<T> from std::vector<T>, accepting all elements. This is needed for gluing things together until there is explicit support in Gaudi. It is instantiated for v1::Track and v2::Track.

Extra minor changes:

• Instantiate PrCheckEmptyFilter for v1::Track and v2::Track

cc: @sstahl @graven @pseyfert