Streamline Documentation w/ modern roxygen2 to avoid repetition and increase consistency

[jack-davison]

The openair documentation has a lot of repetition/redundancy baked into it, so I've tried to streamline it using more modern roxygen2 and lessons from openairmaps. Including:

• Activate markdown syntax and use it where possible.
• Use the "family" tag to group together functions. There are six families I've defined. This automates "seeAlso" (which was inconsistent between functions) and simplifies _pgdown.yml.
  • Polar Directional Analysis
  • Trends and Time Series
  • Trajectory Analysis
  • Import Functions
  • Model Evaluation
  • Cluster Analysis (polarCluster, trajCluster, timeProp - all overlap with other families)
• Use the "inheritParam" tag to avoid repetition and improve consistency. For example, most functions have near-identical "pollution" arguments. Where it makes sense, functions now inherit from a parent function. These are:
  • polarDiff, polarCluster, Annulus, Percentile, Freq inherit from polarPlot
  • trajLevel and trajCluster inherit from trajPlot
  • importSAQN et al. now inherit from importAURN, allowing them to have their own "site" param descriptions
  • conditionalEval inherits from conditionalQuantile
  • pollutionRose inherits from windRose. This means that the documentation can be more bespoke for each and less confusing (no more "for pollutionrose x, for windrose y").
• Used the openair-package.Rd documentation to define the openair class, and then directed all "return" tags to that documentation. Avoids having a three-paragraph description of a list in every single function.
• Where appropriate, used inheritDotParams to make the "..." documentation more descriptive.
• Refactored the documentation for mydata, which used outdated syntax and caused the .Rd file to look weird.
• Collected function imports using usethis::use_import_from() so they weren't dotted around random .R files.
• Removed all instances of data(mydata) which hasn't been needed for a while!
• Various other little fixes to finish half-finished documentation or notes-to-self that never should have been exported

Should pass R CMD check.

[davidcarslaw]

things are sane once more :) thanks!