Apply check functions to fft functions

[yasahi-hpc]

Improves #80

This PR aims at replacing runtime assert with KOKKOSFFT_EXPECTS. Adding more static_assertions if applicable.
Modifications are applied to source code under fft/src and fft/unit_test.

Following modifications are made

• Move KOKKOSFFT_EXPECTS from KokkosFFT_utils.hpp to KokkosFFT_asserts.hpp
• Apply are_valid_axes function where it is applicable
• Apply KOKKOSFFT_EXPECTS to some if else conditional (throw std::runtime_error in the case of else)
• Replacing is_complex<ViewType>::value with is_complex_v<ViewType>

[tpadioleau]

My main question is about whether to enable the checks in non-debug mode or not ?

[yasahi-hpc]

My main question is about whether to enable the checks in non-debug mode or not ?

First of all, thank you for your reviews. Actually, we need it to enable for non-debug mode (different from Kokkos). The point is that if we execute an internal fft library without satisfying the conditions, we may get unexpected errors from the library. I would like to let users know that input data are invalid with the appropriate error messages.