Shorten generation_time_opts() to gt_opts() as alias

[jamesmbaazam]

Description

This PR closes #564.

The generation_time_opts() function was deemed too long and tedious to specify so has been shortened to gt_opts(). Calls to both functions are equivalent and they share the same documentation.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
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After the initial Pull Request

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[jamesmbaazam]

CI errors will be fixed when #700 is merged.

[jamesmbaazam]

Leaving a note here that touchstone has to checkout main to perform the benchmarks against this branch, but main doesn't have gt_opts() yet, so the touchstone script (https://github.com/epiforecasts/EpiNow2/blob/main/touchstone/script.R) here will have to use the long form generation_time_opts() for now and be updated when this PR is merged.

[jamesmbaazam]

CI errors will be fixed when #700 is merged.

This has been resolved but was one of two errors in the CI.

[seabbs]

Lgtm