Fix BEM matrix being added to mass matrix in midpoint predictor

* Explicit timestepper uses get jacobian to get mass matrix. But we have a
  modified get jacobian function to add BEM, so it was getting the BEM as
  well.

* Fixed by adding parameter which is set when entering an explicit step and
  unset after. A bit hacky... but probably ok for now.

* Fix core ooomph-lib way of getting mass matrix later? To prevent this
  happening again.