Codechange: Refactor smooth economy industry production change chances

[davidxn]

Motivation / Problem

The chance of production changes in the smooth economy was based on a multi-layered flipping back and forth of a value called "mult".

Description

Changes this to a straightforward list of chances with a more descriptive name.

Limitations

None

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker