[petsc-dev] I think TS should have a TSSetMassMatrix()

[Barry Smith]

   Then explicit methods could automatically use a solve to apply the mass matrix inverse for each "right hand side" application and implicit methods could automatically "drop in" the mass matrix with the shift when a right hand side Jacobian is given also. Instead of requiring the user to handle this themselves. Are there any gotcha's that I am missing or is this relatively straightforward?

   Why wouldn't we have this interface? 

   Note that adjoints are slightly tricky with a mass matrix without this API.

    Almost for sure MFEM must have this type of API.