fix colour of points in trends

[tomjemmett]

fixes #171 and #122. Currently we colour points based on whether the point is above the mean or below it (taking into account the direction). However, if you have a trend of 7 points that are above the mean line, but all trending down (with a direction of increase being good) previously this would be flagged as improvement when these are infact a decreasing trend.

in resolving this I've also tackled #122 by adding a column which indicates the rule that caused the point to be flagged in the following order

1. if a point is above/below the 3 sigma limits
2. if a point is part of an increasing/decreasing trend
3. if a point is part of a series of 2 in 3 points close to the control limits
4. if a point is part of 7 points one side of the mean

• [x] added unit tests and checked code coverage with covr::report() (should aim for 100%)
• [x] ran devtools::document()
• [x] ran lintr::lint_package() and resolved all lint warnings and notes
• [x] ran styler::style_pkg() to make sure code matches the style guidelines
• [x] ran R-CMD CHECK and resolved all issues

[tomjemmett]

following conversation with @ThomUK at NHS-R conf 23, merging