matrixX_chain: The updated matrixX in the Gibbs sampling process
Description
The updated matrixX in each recorded iteration. An A*B
matrix with each row corresponding to the vectorized matrixX in each
recorded iteration.format
The format is:
num [1:5, 1:150] 0.1878 0.2226 0.1325 0.0274 -0.1524 ...