Parse Restart File for Variable Ordering

[pbauman]

Following the merge of #407, the variable ordering will be dictated by the ordering of the variables in the [Variables] section of the input file. However, it would be nice to be able to 1. use restart files generated before #407 and 2. Be able to use compatible restart files that otherwise have a different variable ordering.

What we can do is parse the restart header information, determine the variable ordering, then internally build up the variables to respect the ordering of the restart file instead of the input file (and throw a warning that we're doing so). This is all fine, but currently there isn't a way to parse only the header information, we'll need to add this capability to libMesh::XdrIO. Then, we should be able to easily implement this capability.