make waitfor() keyword-only

[rerpha]

Description of work

This makes waitfor() keyword-only, which means ISISComputingGroup/IBEX#8850 can never happen.

To test

Which ticket does this PR fix?

Acceptance criteria

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Code Review

• Is the code of an acceptable quality?
• Are there unit tests in place? Are the unit tests small and test the a class in isolation?
• Have the changes been documented in the release notes. If so, do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed.
• How do the changes handle unexpected situations, e.g. bad input?
• Has developer documentation been updated if required?

[Chsudeepta]

If we use g.waitfor(block=1), which is equivalent to the original g.waitfor(1) we will arrive at the same issue as before: 8850