fix: avoid tiny facet panels when plotting many variables

[vrahier]

Since #162, add_facet_wrap() sets the number of facet columns with get_ncol(), which never returns more than 3 columns. Before, ggplot2 chose the layout itself (a near-square grid).

When there are many variables, the layout becomes very tall: 14 variables give 2 columns × 7 rows instead of a 4×4 grid. Scatter plots set aspect.ratio = 1, so the panel size is limited by the plot height and the panels come out tiny. This happens for example with plot_scat_versions_per_sit:

Changes:

• get_ncol(): the 1/2/3-column rule still applies up to 6 facets. Above 6, it uses the ggplot2 default layout (ggplot2::wrap_dims()).
• adjust_legend_position(): the legend goes to the bottom when there are 3 or more columns (ncol >= 3 instead of ncol == 3), since get_ncol() can now return more than 3.

Updated the roxygen docs and the .Rd files.