Added basic tests for pairwise group heatmap + altered sig_stars

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.96 :tada:

Comparison is base (21a5b6b) 80.54% compared to head (5f67819) 85.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #49 +/- ## ========================================== + Coverage 80.54% 85.51% +4.96% ========================================== Files 32 32 Lines 838 842 +4 ========================================== + Hits 675 720 +45 + Misses 163 122 -41 ``` | [Impacted Files](https://codecov.io/gh/nateosher/DM-ME/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [R/lm\_dist.R](https://codecov.io/gh/nateosher/DM-ME/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9sbV9kaXN0LlI=) | `93.10% <ø> (ø)` | | | [R/sig\_stars.R](https://codecov.io/gh/nateosher/DM-ME/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-Ui9zaWdfc3RhcnMuUg==) | `100.00% <100.00%> (+100.00%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://codecov.io/gh/nateosher/DM-ME/pull/49/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) 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)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.