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Gotcha's with Kokkos:

1. Handling legacy C code will take some care. C-style structs -> C++ structs really should have constructors/destructors called, and thus should use "new" and "delete" instead of malloc and free. I think this causes problems with reference counting when Kokkos data types are dropped into legacy C code. Should maybe look into "kokkos_malloc" and "kokkos_free".
2. Kokkos seems to require copy constructors with same syntax as the default copy constructor. Change = delete to = default.
3. Looks like care should definitely be taken regarding static class members. It's delicate in CUDA, and Kokkos reflects this. Move MultiBSplineData into MultiBSpline for now to circumvent this problem.
4. All calls to C++ std library functions in Kokkos parallel regions should be looked at very carefully ("looked at"=purged). Does not play well with CUDA. See "std::fill" in src/Numerics/Spline2/MultiBspline.hpp:evaluate_v(...). Chugs along fine for CPU, GPU code compiles, but frustratingly difficult to find runtime error.
5. In einspline_spo, parallelizing evaluate_v needs "psi" and "einspline" arrays. Overloading the einspline_spo class functor gives access to these views, whereas using the provided KOKKOS_LAMBDA macro assumes the Lambda function will capture by value, NOT by reference which is required.