Changes to adapt to Expr changes.
Reorganization and start of Sundials support.
Partially working Sundials solver plus fixed the DASSL solver.
Fix some segfaults.
Convert the dassl solver back to using a global variable.
Tweaks and fixes to the sundials solver.
Keep dasslsim as the default.
Readme updates.
Change ref/assign to getindex/setindex!.
Remove depreciated/removed Base functions.