Add device-friendly numeric_limits-like struct

[bartgol]

It happens often that one needs stuff from std::numeric_limits<ScalarT>, like epsilon or max. Unfortunately, those methods are not device friendly. EKAT could implement a device friendly version of those, which defaults to std::numeric_limits on host.

This would allow to avoid several nvcc warnings of the form

warning: calling a constexpr __host__ function("min") from a __host__ __device__ function("operator()") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.