Workaround for matrix-free BEuler in Python

Hong Zhangrequested to merge
hongzh/mf-beuler into release
  • petsc4py uses the "Python" matrix class for a matrix-free implementation. Unlike MatShell, support for MatScale and MatShift is missing in petsc4py. As a workaround, this commit removes MatScale in BEuler and scales the vector to be multiplied instead.
  • JacobianP evaluation in RK is needed only when b is nonzero.

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