Update algorithm for mass matrix.
     
    
    Usage
    .rotate_posterior(metric, fn, gr, Q, Qinv, y.cur)
 
     
    
    Arguments
- metric
 
The metric to use
 
- fn
 
The current fn function.
 
- gr
 
The current gr function
 
- Q
 
The sparse precision matrix
 
- Qinv
 
The inverse of Q
 
- y.cur
 
The current parameter vector in unrotated (Y) space.