cmake: Fix inconsistent thrust detection across the backends

[stotko]

CUDA ships the thrust library by default. Since it lives in the same include directory as the CUDA headers, the detection for the CUDA backend implicitly works. However, this prevents pointing to another (e.g. more recent) version of thrust. Furthermore, no version check is performed to ensure that the requirements are fulfilled. Change that to find thrust for all backends consistently. In addition, explicitly set the device backend for thrust when using CUDA to make this consistent to the OpenMP backend and become independent of thrust's default backend choice.

[codecov]

Codecov Report

Merging #92 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #92   +/-   ##
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  Coverage   97.82%   97.82%           
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  Files          27       27           
  Lines        1611     1611           
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  Hits         1576     1576           
  Misses         35       35

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