Make states fill_color="none" (transparent)

[ahuang11]

Closes #512 (not sure why I closed the previous PR; perhaps because it was stale).

Also not sure if it's a better choice to set fill_alpha vs fill_color. I leaned towards fill_color rather than fill_alpha because I worry users might be confused if they change fill_color and it doesn't show anything.

I would like to skip importing and calling geoviews:

import pandas as pd
import hvplot.pandas
import geoviews as gv

df = pd.read_html(
    "https://mesonet.agron.iastate.edu/sites/networks.php?station=ORD&network=IL_ASOS"
)[1]
# make snake case
df.columns = df.columns.str.lower().str.replace(" ", "_")

map = df.hvplot.points(
    "longitude1",
    "latitude1",
    geo=True,
)

# overlay the map
states = gv.feature.states(fill_color="none")
map * states

And instead, do this:

import pandas as pd
import hvplot.pandas

df = pd.read_html(
    "https://mesonet.agron.iastate.edu/sites/networks.php?station=ORD&network=IL_ASOS"
)[1]
# make snake case
df.columns = df.columns.str.lower().str.replace(" ", "_")

map = df.hvplot.points(
    "longitude1",
    "latitude1",
    geo=True,
    features=["states"]
)

# overlay the map
map

At the moment, it shows this:

[hoxbro]

If you update the CHANGELOG.md and doc/releases.rst with a comment about this PR today, I will add this to the next release. Maybe a small comment about this is a (small) breaking change.

Using color makes sense to me. That will be the first thing coming to mind when wanting a background color, not the alpha.

See here: https://github.com/holoviz/geoviews/pull/637/files

[ahuang11]

Okay done; feel free to make direct edits if needed!

[hoxbro]

Thank you for the quick turnaround.

[ahuang11]

Thanks for getting this in! Can't wait to use it in hvplot!