Add csqrt, cpow to cuda_complex.hpp

8 jobs for csqrt in 5 minutes and 43 seconds (queued for 3 seconds)
Name Stage Failure
tests-and-coverage Test
14.91s call     pystencils_tests/
12.02s call pystencils_tests/
11.58s call pystencils_tests/test_small_block_benchmark.ipynb::test_small_block_benchmark.ipynb
===== 14 failed, 256 passed, 5 skipped, 329 warnings in 145.98s (0:02:25) ======
Uploading artifacts...
coverage_report: found 73 matching files
test-report: found 4 matching files

Uploading artifacts to coordinator... ok
id=335543 responseStatus=201 Created token=daPjzc48
ERROR: Job failed: exit code 1