[el2p, multidomain] fix residual calculation, generalize multidomain

convergence writer

- The residual calculation for el2p was wrong, probably never used. It now
  forwards the call to the GridOperator functionality.

- The multidomain convergence writer assumed that sub-domain solution
  vectors can be default-contructed. This is not the case if a PDELab
  model is used for the subdomain.

Reviewed by Dennis.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14200 2fb0f335-1f38-0410-981e-8018bf24f1b0