Added tests for remaining plotting functions

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +7.15 :tada:

Comparison is base (38c8d4a) 85.66% compared to head (50f262f) 92.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #74 +/- ## ========================================== + Coverage 85.66% 92.82% +7.15% ========================================== Files 32 32 Lines 879 892 +13 ========================================== + Hits 753 828 +75 + Misses 126 64 -62 ``` | [Impacted Files](https://codecov.io/gh/nateosher/DM-ME/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [R/group\_boxplots.R](https://codecov.io/gh/nateosher/DM-ME/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9ncm91cF9ib3hwbG90cy5S) | `100.00% <100.00%> (+100.00%)` | :arrow_up: | | [R/plot\_dist.R](https://codecov.io/gh/nateosher/DM-ME/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9wbG90X2Rpc3QuUg==) | `72.72% <100.00%> (+72.72%)` | :arrow_up: | | [R/plot\_intensities.R](https://codecov.io/gh/nateosher/DM-ME/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9wbG90X2ludGVuc2l0aWVzLlI=) | `100.00% <100.00%> (+100.00%)` | :arrow_up: | | [R/plot\_ppp.R](https://codecov.io/gh/nateosher/DM-ME/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9wbG90X3BwcC5S) | `100.00% <100.00%> (+100.00%)` | :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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