Calling host functions/operators from function with device annotation

[pgrete]

There are several places in the code where host/device annotation (e.g., through KOKKOS_INLINE_FUNCTION) result in host called that is potentially called from a device, e.g.,

/home/pgrete/src/parthenon/src/mesh/domain.hpp(67): warning: calling a __host__ function("std::array< ::parthenon::IndexRange, (unsigned long)3ul> ::operator []") from a __host__ __device__ function("parthenon::IndexShape::MakeZeroDimensional_") is not allowed

/home/pgrete/src/parthenon/src/mesh/domain.hpp(68): warning: calling a __host__ function("std::array<int, (unsigned long)3ul> ::operator []") from a __host__ __device__ function("parthenon::IndexShape::MakeZeroDimensional_") is not allowed

/home/pgrete/src/parthenon/src/interface/variable_pack.hpp(62): warning: calling a __host__ function from a __host__ __device__ function is not allowed
          detected during instantiation of "int parthenon::VariablePack<T>::GetDim(int) [with T=parthenon::Real]" 
/home/pgrete/src/parthenon/tst/unit/test_container_iterator.cpp(93): here

As far as I can tell this is currently not a problem because we actually never call these function from within a kernel, but this should nevertheless be addressed.
I started to simplify the IndexRange struct in #165 as it caused the majority of warnings.

Anyone, feel free to pick this up (I'm still working on #165 and #136 and want to finish those with high priority).

[JoshuaSBrown]

Ah, this is my fault. I should try my hand at fixing it.