mpem 0.1.0

• Initial standalone package for matrix partial EM (MPEM).
• Added one public mpem() interface for incomplete Kronecker-structured matrix-normal data.
• Added automatic dispatch between general MPEM for arbitrary missingness and Rect-MPEM for rectangular structural missingness.
• Returned imputed data and fitted parameters directly in the mpem_fit object.
• Added finite mixtures of matrix-normal distributions directly to mpem() through G > 1, with weighted general MPEM and weighted Rect-MPEM updates.