call operator of ProjFunctor must be const

[oleksandr-pavlyk]

Also changed the implementation of ProjFunctor to make compiler's job easier.

const is required for this struct to be usable in kernel if it is created on the host and passed to the kernel as parameter.

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• Have you added a test, reproducer or referred to an issue with a reproducer?
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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1415/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.15.0rc1=py310ha25a700_10 ran successfully.
Passed: 916
Failed: 84
Skipped: 119

[coveralls]

coverage: 85.774%. remained the same when pulling b77720b on add-missing-const into 1f34c0d on master.

[github-actions]

Array API standard conformance tests for dpctl=0.15.0rc1=py310ha25a700_12 ran successfully.
Passed: 916
Failed: 84
Skipped: 119

[ndgrigorian]

@oleksandr-pavlyk similar to my PR, you may want to update pyproject.toml again to reflect broken_complex skips on all platforms

"broken_complex: mark a test that is skipped on Windows due to complex implementation"

[github-actions]

Array API standard conformance tests for dpctl=0.15.0rc1=py310ha25a700_13 ran successfully.
Passed: 916
Failed: 84
Skipped: 119

[ndgrigorian]

Windows CI is still red for some complex64 trig function tests, but we can add another PR to skip those.

This LGTM, thank you @oleksandr-pavlyk

[github-actions]

Array API standard conformance tests for dpctl=0.15.0rc1=py310ha25a700_15 ran successfully.
Passed: 916
Failed: 84
Skipped: 119