Adds additional interface for linear POD

adds `OneLevelTransport` member functions for extracting separate Jacobian components to allow @MajinSaha 's implementation of POD for linear equations. 

The changes are relatively minor, but may be unneeded eventually depending on how we handle general nonlinear problems.

`test/POD` directory contains an example of POD for a linear 3D heat equation



 
Yes, that's right.