atomic: Make all operations follow sequentially consistent ordering

[stotko]

Up to now, atomic and atomic_ref do not specify the memory order that they will enforce. In the C++ standard library, the default memory order for calls to them is sequential consistency, so users will assume that stdgpu also enforces this order. However, we did not explicitly enforced this which might lead to runtime errors or dead locks. Explicitly make all atomic member functions, including load() and store() follow sequentially consistent ordering to ensure consistency with the C++ standard library.

[codecov]

Codecov Report

Merging #176 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   97.80%   97.81%           
=======================================
  Files          28       28           
  Lines        1825     1833    +8     
=======================================
+ Hits         1785     1793    +8     
  Misses         40       40           

Impacted Files	Coverage Δ
src/stdgpu/impl/atomic_detail.cuh	98.44% <100.00%> (ø)
src/stdgpu/impl/bitset_detail.cuh	100.00% <100.00%> (ø)
src/stdgpu/openmp/impl/atomic_detail.h	100.00% <100.00%> (ø)

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