ggplot2 implementation of many openair functions

[jack-davison]

This PR implements ggplot2 versions of various plotting functions, as part of the drive to provide a modern API for openair.

Time Series & Trends

• trendLevel() -> plot_heatmap()
• calendarPlot() -> plot_calendar()
• timeProp() -> plot_trend_bars()
• timePlot() -> plot_trend_lines()
• smoothTrend() -> plot_trend_smooth()
• TheilSen() -> plot_trend_theilsen()
• timeVariation() -> plot_variation()

Directional Analysis

• polarFreq() -> plot_polar_heatmap()
• percentileRose() -> plot_polar_percentile()

Model Eval

• conditionalQuantile() -> plot_conditional_quantile()
• taylorDiagram() -> plot_taylor_diagram()

Other Plots

• scatterPlot() -> plot_xy_scatter() and plot_xy_bins()
• corPlot() -> plot_correlation()

This PR also adds some necessary structural changes to initialise ggplot2 in openair:

• Adds ggplot2 (>=4.0.0) and scales as dependencies.
• Adds exported "options" functions to help manage options in a consistent way:
  • discretise_opts for flexibly discretising a scale (e.g., plot_calendar, plot_heatmap)
  • scale_opts for scale controls (limits, transforms, breaks)
  • facet_opts for controlling how facets are handled
  • windflow_opts for controlling the appearance of windflows
• Adds label_openair(), which is a vectorised version of quickText() more suitable for use in ggplot2.

Old functions are still present, and documentation cross-links them.