location-scale Student t distribution

[mjskay]

This PR converts dist_student_t to location-scale form. It closes #28.

Specific changes:

• Changes the constructor signature from dist_student_t(df, ncp = NULL) to dist_student_t(df, mu = 0, sigma = 1, ncp = NULL)
• Converts the cdf, quantile, density, generate, mean, and variance functions to location-scale form
• Adds tests for the noncentral t, location-scale t, and noncentral location-scale t distributions. Not sure anyone ever uses that last combination, but it seemed simpler to allow it than to add a special-case to disallow it :).
• Change the printing format for the t distribution to be t(df, mu, sigma) when ncp is NULL and t(df, mu, sigma, ncp) when ncp is not NULL. I'm not sure if that's what you'd like but it was the best option I could think of. Let me know if you'd rather something else.

[codecov-commenter]

Codecov Report

Merging #38 into master will increase coverage by 0.63%. The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   74.78%   75.42%   +0.63%     
==========================================
  Files          43       43              
  Lines        1063     1066       +3     
==========================================
+ Hits          795      804       +9     
+ Misses        268      262       -6     

Impacted Files	Coverage Δ
R/dist_student_t.R	77.77% <88.46%> (+20.20%)	:arrow_up:

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[mjskay]

Sure, will do both now