Added alpha_fd coefficient for Beljaars TOFD in GSL drag

[kayeekayee]

Description

Added alpha_fd coefficient for Beljaars TOFD in GSL drag

Type of change

• New feature (adds functionality)

Change characteristics

• Is this a breaking change (a change in existing functionality)? NO
• Does this change require a documentation update? NO

How has this been tested?

Tested on Hera

Checklist

• [x ] Any dependent changes have been merged and published
• [x ] My code follows the style guidelines of this project
• [x ] I have performed a self-review of my own code
• [x ] I have commented my code, particularly in hard-to-understand areas
• [x ] My changes generate no new warnings
• [x ] New and existing tests pass with my changes
• [x ] I have made corresponding changes to the documentation if necessary