Allow users to add new Control classes without implementing a plugin

[hansthen]

@Conengmo

This is a new, smaller implementation of the Control class I implemented earlier. For this PR I focused only on the user's ability to load a new control in user space, without having to write a new plugin. It is not difficult to write a plugin, but it does require knowledge of Jinja. Also, there is quite some boilerplate code involved.

With this Control clas I was able to implement the minimap and ruler plugins, using just a few lines of code.

import folium
import streamlit as st
from folium import Control, TileLayer

from streamlit_folium import st_folium

st.set_page_config(
    page_title="minimap",
    layout="wide"
)

m = folium.Map(
    location=[39.949610, -75.150282],
    zoom_start=5,
    zoom_control=False
)
tiles = TileLayer(
    tiles="OpenStreetMap",
    show=False,
    control=False,
)
tiles.add_to(m)

minimap = Control("MiniMap", tiles)
minimap.add_js_link(
    "minimap_js",
    "https://cdnjs.cloudflare.com/ajax/libs/leaflet-minimap/3.6.1/Control.MiniMap.min.js"
)
minimap.add_css_link(
    "minimap_css",
    "https://cdnjs.cloudflare.com/ajax/libs/leaflet-minimap/3.6.1/Control.MiniMap.css"
)
minimap.add_to(m)

ruler = Control("Ruler", tiles)
ruler.add_js_link(
    "ruler_js",
    "https://cdn.rawgit.com/gokertanrisever/leaflet-ruler/master/src/leaflet-ruler.js",
)
ruler.add_css_link(
    "ruler_css",
    "https://cdn.rawgit.com/gokertanrisever/leaflet-ruler/master/src/leaflet-ruler.css",
)
ruler.add_to(m)

st_folium(m, width=1600, height=600)

[hansthen]

Closes #1992

[hansthen]

@ocefpaf can you have a look?

[ocefpaf]

One problem that we have in folium templates are the silent erros when passing keywords to it. For example, if someone types bottonright instead of bottomright, it will take user a while to figure out what is wrong.

Maybe we should check for this 4 valid kw? I know this doesn't solve it for all templates, but in this case it may be small enough that it is worth it.

[hansthen]

Do you mean as a static typing check or as an assert?

[ocefpaf]

We should avoid assert use outside of tests.

I'm old school but here is what I would do:

1. make position a kw argument, not an arg;
2. Sanitize it with a position.lower() to validate names copied from leaflet JS docs;
3. Check if the sanitized positions is in a valid list with those 4 names and raise a ValueError if not.

What do you think?

[hansthen]

Makes sense.

[hansthen]

I see we have been inconsistent in banning assert :-)

[ocefpaf]

While I was a Fortran programmer in my early days, type checking is not my forte. (Heck, I moved to Python to get away from this ;-p)

My crude review tells me that this is equivalent to what I described in my review comment, right? If so, merge away!

[hansthen]

I wrote my first program for money in RPG :-) which is a fairly obscure language almost as old as Fortran. Whitespace is significant to a fault.

     C* PARAMETER LIST
     C           *ENTRY    PLIST
     C                     PARM           DATEIN  80
     C* PROGRAM START
     C           DATEIN    DIV  100       DATEIN
     C                     MVR            DAY     20
     C           DATEIN    DIV  100       DATEIN
     C                     MVR            MONTH   20
     C                     Z-ADDDATEIN    YEAR    40
     C* SHOW RESULTS
     C           YEAR      DSPLY
     C           MONTH     DSPLY
     C           DAY       DSPLY
     C                     SETON                         LR
     C* EXECUTION EXAMPLE
     C* CALL SEPDATE PARM(X'019960531F')

I added one more test.