unequal weights and informative messaging in optimize_design()

[zabore]

This pull request is related to two open issues with optimize_design():

1. Adds informative messaging when no results are returned from optimize_design() due to the filtering by type I error and/or power
2. Adds an option for unequal weighting to optimize_design()

Closes #14 and Closes #7

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How-to Checklist:

• Create an issue in GitHub
• Use GitHub desktop to create a new branch
• Open ppseq R project and make sure it is on the right branch for the issue
• Make changes to code
• Run R CMD CHECK and fix any warnings or errors
• Run pkgdown::build_site()
• Commit changes to the right branch for the issue
• Use GitHub desktop to publish the new branch to GitHub
• Use GitHub desktop to create a pull request associated with the new branch
• On GitHub, add detailed comments to the pull request. Make sure to mention the # of the GitHub issue using keywords to close the issue automatically once the PR is merged (e.g. "Closes #1112")
• Increment the version number using usethis::use_version(which = "dev") (or a different argument to which, if appropriate)
• Update NEWS.md with the changes from this pull request under the heading "# ppseq (development version)". If there is an issue associated with the pull request, reference it here.
• Commit to the right branch for the issue and push to GitHub
• Merge the PR using "Squash and merge" once all checks have passed
• Delete both the local and remote branches