v4.0.0

PreprocessMD v4.0.0

Diff since v3.1.1

Closed issues:

• Generate cohorts from concept sets (#201)
• Add doctests for generate_cohort() (#204)
• Change MLJ.jl from a dependency to a weakdep (#211)

Merged pull requests:

• added generate_cohort function (#202) (@Mounika-Thakkallapally)
• Add doctest (#205) (@AshlinHarris)
• Update CONTRIBUTING.md (#206) (@AshlinHarris)
• Add subset_invalid_date (#209) (@AshlinHarris)
• Include MLJ as a weakdep (#214) (@AshlinHarris)
• Add compat (#215) (@AshlinHarris)