multivariate mixture
"EII"   =   spherical, equal volume
"VII"   =   spherical, unequal volume
"EEI"   =   diagonal, equal volume and shape
"VEI"   =   diagonal, varying volume, equal shape
"EVI"   =   diagonal, equal volume, varying shape
"VVI"   =   diagonal, varying volume and shape
"EEE"   =   ellipsoidal, equal volume, shape, and orientation
"EVE"   =   ellipsoidal, equal volume and orientation (*)
"VEE"   =   ellipsoidal, equal shape and orientation (*)
"VVE"   =   ellipsoidal, equal orientation (*)
"EEV"   =   ellipsoidal, equal volume and equal shape
"VEV"   =   ellipsoidal, equal shape
"EVV"   =   ellipsoidal, equal volume (*)
"VVV"   =   ellipsoidal, varying volume, shape, and orientation