Add IMM vignette, example data, and updated distribution functions

[GidonFrischkorn]

This commit adds the vignette for the IMM. Contains two example data sets, and has also included density and random generation functions for the mixture2p, the mixture3p, and the IMM.

Summary

• Updated the distribution functions for mixture2p, mixture3p, and IMM
• Added ZhangLuck_2008 and OberauerLin_2017 datasets with documentation
• Add vignette for the IMM

Tests

[x] Confirm that all tests passed
[x] Confirm that devtools::check() produces no errors

Release notes

• Updated the distribution functions for mixture2p, mixture3p, and IMM
• Added ZhangLuck_2008 and OberauerLin_2017 datasets with documentation
• Add vignette for the IMM

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (06a963c) 64.79% compared to head (17add92) 65.84%.

Files	Patch %	Lines
R/distributions.R	73.68%	30 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #69      +/-   ##
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+ Coverage    64.79%   65.84%   +1.04%     
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  Files           16       16              
  Lines          855      969     +114     
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+ Hits           554      638      +84     
- Misses         301      331      +30     

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[venpopov]

Awesome work. My only suggestion is to include the figure from Oberauer that you refer to - it's best that the vignette is self-contained. I'll aprove and merge the pull request, and you can push the updated Rmd vignette directly to develop, if you don't change any other code but just add the figure.