state.to_spec should validate if type is correct

[hoxbro]

Right now, state.to_spec will run the respective class to_spec method. There should be a type check to validate that the type is correct, e.g., layouts should have the type of Layouts:

import lumen
import panel as pn
import yaml
from lumen.layout import Layout
from lumen.pipeline import Pipeline
from lumen.sources import FileSource
from lumen.views import Table

pn.extension("tabulator")

data_url = "https://datasets.holoviz.org/penguins/v1/penguins.csv"

pipeline = Pipeline(source=FileSource(tables={"penguins": data_url}), table="penguins")
pipeline.add_filter("widget", field="species")
pipeline.add_filter("widget", field="island")
pipeline.add_filter("widget", field="sex")
pipeline.add_filter("widget", field="year")

view = Table(pipeline=pipeline)

This should raise an error

specification = lumen.state.to_spec(
    config={"title": "test"},
    layouts=[view],
)

I can submit a PR, but I want to be sure I have not overlooked anything.

[philippjfr]

Maybe also a good idea for lumen.state.to_spec to also run a validation pass on the output it generates.