alpha For Input into fit_lda_cThere are a bunch of ways users could format alpha but the C++ Gibbs
sampler in fit_lda_c only takes it one way. This function does the
appropriate formatting. It also returns errors if the user input a malformatted
alpha.
format_alpha(alpha, k)Returns a list with two elements: alpha and alpha_class.
alpha is the post-formatted version of alpha in the form of a
k-length numeric vector. alpha_class is a character
denoting whether or not the user-supplied alpha was a "scalar" or
"vector".
the prior for topics over documents. Can be a numeric scalar or numeric vector.
the number of topics.