enh: Add select to ImageStack

[hoxbro]

Improves the issue described here: holoviz-topics/examples#430 (comment)

The current implementation is pretty rough around the edges...

import datashader as ds
import holoviews as hv
import numpy as np
import pandas as pd
from holoviews.operation.datashader import rasterize

hv.extension("bokeh")

num = 10000
np.random.seed(1)

dists = {
    cat: pd.DataFrame({
        "x": np.random.normal(x, s, num),
        "y": np.random.normal(y, s, num),
        "s": s,
        "val": val,
        "cat": cat,
    })
    for x, y, s, val, cat in [
        (2, 2, 0.03, 0, "d1"),
        (2, -2, 0.10, 1, "d2"),
        (-2, -2, 0.50, 2, "d3"),
        (-2, 2, 1.00, 3, "d4"),
        (0, 0, 3.00, 4, "d5"),
    ]
}

df = pd.concat(dists, ignore_index=True)

points = hv.Points(df)
plot = rasterize(points, aggregator=ds.count_cat("cat"))

plot + plot.select(cat=["d1", "d3"])

[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.

Project coverage is 88.49%. Comparing base (366d932) to head (7233cc2).

Files with missing lines	Patch %	Lines
holoviews/element/raster.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6437      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.02%     
==========================================
  Files         323      323              
  Lines       68627    68636       +9     
==========================================
+ Hits        60737    60738       +1     
- Misses       7890     7898       +8     

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