Make calculation of p-vals options in get_pairwise_comparisons()

[nikosbosse]

Description

This PR closes #978

As suggested in #978, this PR adds an option to make the computation of p values optional.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
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[seabbs]

do we need to move to test on 4.1 the error is not knowing about the base pipe

[sbfnk]

(need to then also update DESCRIPTION)

[elray1]

the changes themselves look good to me. approving (subject to other comments from Sam and Seb)

[nikosbosse]

switched back to the old pipe