Updates based on Scott's first review

Since we don't have working CI, let's paste our test results in the PR:

> checking top-level files ... NOTE
  Non-standard file/directory found at top level:
    '2018InstSites.csv'

0 errors v | 0 warnings v | 1 note x

R CMD check succeeded

And the code coverage:

covr::package_coverage()
HASP Coverage: 83.36%
R/explore.R: 0.00%
R/get_data.R: 0.00%
R/analyze.R: 88.66%
R/chloride.R: 92.41%
R/frequency_analysis.R: 94.39%
R/Sc_Cl_plot.R: 95.04%
R/statistics.R: 95.74%
R/gwl_single_sites.R: 97.37%
R/ggplot2_utils.R: 100.00%
R/included_data.R: 100.00%
R/visualize.R: 100.00%