improve handling of identity or constant mass matrix

[martinjrobins]

mass matrix currently implemented via an implicit function $Mv$, for a large majority of cases $M=I$ or $M$ is constant, so I think we need to factor in efficiencies for these cases

• Should the solvers be aware if $M=I$ or not and act accordingly? Or should the efficiencies be implemented in OdeEquations?
• for the latter above, should OdeEquations provide a gemv style function for the implicit solvers to use?
• and an $M^{-1}$ function for the explicit solvers?