Feature/load-factor-improvements

[roger-teoh]

Closes #XX

Changes

• Adds function to estimate aircraft (passenger/cargo) load factor based on historical monthly and regional load factors provided by IATA. This improves upon the default load factor assumption.
• Historical load factor databases will be continuously updated as new data is released.

Breaking changes

Features

Fixes

Internals

• Update default load factor from 70% to 83% to be consistent with historical data

Tests

• QC test passes locally (make test)
• CI tests pass

Reviewer

Select @ reviewer

[zebengberg]

@roger-teoh -- I'll make a few small commits to this branch to avoiding reading the csvs whenever the jet module is imported.