Only Reinit Variables That are Used During PP Eval

[friedmud]

Currently if we have 3000 variables but we have only one ElementPostprocessor active and it only uses one of those 3000 variables we still evaluate all of them at the quadrature points!

Need to only reinit the variables that are going to be used at the quadrature points.

[friedmud]

In 70e94ae:

add new base classes that keep track of objects dependency on MooseVariables and switch all of the systems over to it ref #1386

[friedmud]

In d3fdaf5:

Only reinit variables that are being used.  This will greatly speed up applications with a large number of variables by only computing those variables when they are needed.  closes #1386

[permcody]

Re-opening the ticket cause I know how much you love it ;-)

You need to fix all the indexing into zero length container errors present in Elk and Fox since d3fdaf5

[friedmud]

In c81f652:

fixing corner case closes #1386