Add functionality and improvements around design text file

[bvreede]

• [x] Improve the layout of the design file
• [ ] Add a function that verifies its contents
• [x] Add a function that creates a design
• [x] (optional) Have design file created with Q&A with user

Closes #14, closes #16

[ankemt]

Codecov Report

Patch coverage: 86.56% and project coverage change: -0.98 :warning:

Comparison is base (6ab47de) 87.20% compared to head (0d3d617) 86.23%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #33 +/- ## ========================================== - Coverage 87.20% 86.23% -0.98% ========================================== Files 6 7 +1 Lines 172 218 +46 ========================================== + Hits 150 188 +38 - Misses 22 30 +8 ``` | [Impacted Files](https://codecov.io/gh/ankemt/MEAanalysis/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [R/treatment\_ratio.R](https://codecov.io/gh/ankemt/MEAanalysis/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui90cmVhdG1lbnRfcmF0aW8uUg==) | `100.00% <ø> (ø)` | | | [R/exp\_design.R](https://codecov.io/gh/ankemt/MEAanalysis/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9leHBfZGVzaWduLlI=) | `81.63% <81.63%> (ø)` | | | [R/parse\_designfile.R](https://codecov.io/gh/ankemt/MEAanalysis/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9wYXJzZV9kZXNpZ25maWxlLlI=) | `100.00% <100.00%> (+2.85%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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