Install future and future.apply to fix R CMD CHECK

[jamesmbaazam]

Description

This PR closes #826. Since #798, the future and future.apply packages were made optional but future is required to run the tests, so R CMD CHECK has been failing on main and subsequent PRs (#774, #825). This PR fixes the failing R CMD CHECK on main by installing future. It additionally installs future.apply so that it can be tested instead of using lapply().

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.