Use the correct mesh for output in misc ex10
write_equation_systems de-inlining, extra assert

The extra assert catches the (just-fixed) bug in misc ex10.

The de-inlining seemed more sensible than adding another
equation_systems.h dependency, since this is a costly method to begin
with.
Merge pull request #281 from libMesh/double_check_mesh

Double check mesh in write_equation_systems