Converts a vector of membership IDs into a sparse matrix
     
    
    Usage
    cluster_membership_matrix(groups, group_order = NULL)
 
     
    
    Arguments
- groups
 
Vector with one entry per cell, specifying the cell's group
 
- group_order
 
Optional vector listing ordering of groups
 
 
    
    Value
    cell x group matrix where an entry is 1 when a cell is in a given group