Prepare RELAP-7 for MooseVariable::sln() const update.
Prepare RELAP-7 for MooseVariable::sln() const update.

This set of changes prepares RELAP-7 to work with an upcoming version of
MOOSE in which MooseVariable::sln() returns a const reference.

Refs idaholab/moose#6327.

bwspenc/moose,laagesen/moose,idaholab/moose,harterj/moose,laagesen/moose,andrsd/moose,sapitts/moose,lindsayad/moose,sapitts/moose,dschwen/moose,andrsd/moose,jessecarterMOOSE/moose,laagesen/moose,lindsayad/moose,bwspenc/moose,lindsayad/moose,jessecarterMOOSE/moose,lindsayad/moose,idaholab/moose,milljm/moose,dschwen/moose,laagesen/moose,harterj/moose,andrsd/moose,bwspenc/moose,bwspenc/moose,jessecarterMOOSE/moose,dschwen/moose,sapitts/moose,harterj/moose,harterj/moose,sapitts/moose,milljm/moose,andrsd/moose,idaholab/moose,dschwen/moose,milljm/moose,milljm/moose,dschwen/moose,idaholab/moose,jessecarterMOOSE/moose,bwspenc/moose,laagesen/moose,idaholab/moose,jessecarterMOOSE/moose,harterj/moose,andrsd/moose,lindsayad/moose,milljm/moose,sapitts/moose