Add pair_plot_focus

[OriolAbril]

Talking with @daniel-saunders-phil he mentioned he often wanted to generate pair plots of a specific variable against several others. I think we could have a function that makes that easy (and paves the way with visuals and computation for plot_pair once we have the PlotMatrix manager) because as we compare multiple variables to always the same one the facetting/mapping logic is the one in PlotCollection.

I was thinking the API would be similar to a regular plot with dt, group, var_names... and then a target (or similar) name where you provide either a string (var_name in dt input) or a DataArray.

---

pseudo example

from arviz_base import load_arviz_data
from arviz_base.labels import BaseLabeller, NoVarLabeller
import arviz_plots as azp

azp.style.use("arviz-clean")
plt.rcParams["font.size"] = 8  # clean theme sets font too big

idata = load_arviz_data("centered_eight")
target_name = "mu"
pc = azp.PlotCollection.wrap(
    idata.posterior.ds[["theta"]],
    cols=["school"],
    col_wrap=4,
    plot_grid_kws={"figsize": (8, 3), "sharey": True},
)
pc.map(
    azp.visuals.scatter_x,
    "samples",
    y=idata.posterior[target_name],
    marker=".",
    alpha=.4,
    edgecolor="none",
)
pc.map(
    azp.visuals.labelled_title,
    "title",
    subset_info=True,
    labeller=NoVarLabeller(),
    ignore_aes={"color"},
    size=8,
)

pc.viz["chart"].item().suptitle(f"Pair plots of theta subsets vs {target_name}");