Session file format v2
It's become more apparently that we need more flexibility in the input file format, so this issue is a place to evolve that.
Requirements:
- more flexibility in defining solvers, preconditioners, solution methods, etc.
- defining configuration for multiple equation systems
- removing dependence on having a fully-populated XML file for setting up solvers (#165 (closed))
Discussion points:
- How to maintain support for v1 session files & backwards compatibility
- Restructuring of the SessionReader to accommodate this