Deform excisions with Lorentz boost

[nilsvu]

To use boosted Kerr black holes as conformal background in our initial data we have to deform the excision surfaces with a Lorentz boost. Currently we only deform the excisions to conform to unboosted Kerr horizons. These are the steps that need to be done to add the boost:

1. One way to compute the boosted horizon shape is to add the boost to gr::Solutions::kerr_schild_radius_from_boyer_lindquist() (see Sec. 4 of https://arxiv.org/pdf/0706.0622).
2. Add the boost velocity as an option to src/Domain/Creators/TimeDependentOptions/ShapeMap.hpp:30 and use it in src/Domain/Creators/TimeDependentOptions/ShapeMap.cpp:80.
3. Add a test to confirm that the boosted shape map indeed conforms to a boosted Kerr horizon.

Note that SpEC implements a coordinate map called RescaleAxesOblique to do this. We don't need to do that, since we can just use the shape map to deform the excision. But the code in SpEC could be helpful to look at.

[shabibti]

I've just opened a PR for this here: #6363