Elements table generator

[bmaranville]

Moves the table generator from periodictable/nsf.py to here in standalone util/scattering_table_html.py utility.

Also adds lowercase element symbols as target tags, for compatibility with the old pages (and redirects).

Adds a scroll offset so the target element is properly in view.

[pkienzle]

Add periodictable version number to the bottom of the table.

Put the table in the activation calculator html directory and link to it from the help text. I link to a resonances table from the "Source neutrons" help, so we could put it beside that.

Add the table deploy command to the pyodide deploy script.

Move scripts to a helper directory?

I would leave the table generation code in the periodictable repo, but I won't veto moving it.

[bmaranville]

If we always deploy this and the activation calculator together, it just beomes one script, which is nice. The plan is to redirect pages like n-lengths/pt.html to scattering_table.html#pt

[pkienzle]

If we link from the help, won't we need a deploy script for the table when setting up the flask version? The flask instructions will need to include this step.

The reason for keeping the html generation with periodictable is if we believe others will want to generate their own tables. If it is only needed for the activation pages then it doesn't matter where it lives.

[bmaranville]

The use case for a custom table and the use case for embedding the activation calculator don't overlap, do they? I can't think of why we wouldn't link to a canonical URL for the scattering table instead of a local one.