Made the default gravity vector use the default gravity magnitude
Made the default gravity vector use the default gravity magnitude

Before, the default gravity vector used its own magnitude, 9.8,
instead of the default gravity magnitude, 9.81. Thus, there was
redundancy and inconsistency. Tests were re-golded as necessary
to account for the difference of 0.01 in the gravity constant.

Refs idaholab/relap-7#555

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