Team prediction improvements

[nivaldoh]

• speed up team validation: reuse a single showdown-validator process instead of recreating it for each item
• improve usage_stats agreggation: enable filtering data by Smogon rank baseline
  • Rank has been added as a subdirectory for the Pokémon usage jsons created: e.g. "checks_data/gen1/ou/{RANK}/2014-12.json"
  • rank is a new argument for usage_stats, so it had to be added to existing calls of usage_stats
  • rank has a default value. If a rank that doesn't exist as a Smogon file is selected (e.g 1501), the nearest lower available rank will be picked instead (e.g. 1500).
• scraping data from Smogon was failing for some files apparently due to time out, so exponential backoff + concurency limit were added to fix this