1. Update ARKode's handling of time-dependent mass matrices -- the
   documentation has been updated to reflect the correct approach, I
   just need to implement this in code.

2. Examine restructure where we solve for and store the scaled RHS
   vectors instead of the stages themselves, as discussed in various
   references as being generally preferred for very stiff problems.

3. Revisit scaling vectors with PCG (without preconditioning) -- this
   seems to work improperly now.  Perhaps it is the testing code, or
   maybe the solver itself.
  
5. idaFoodWeb_kry_omp had been explicitly disabled from our testing
   (in the examples/ida/C_openmp/CMakeLists.txt file).  Why?  It runs
   correctly, although my results differ from the 'answer' file.
   Perhaps this problem should be investigated more thoroughly and
   added back in? 
  
6. Revisit the question of the differencing parameter used in the
   IDALs and IDASLs interfaces.  Previously we had different
   increments for GMRES than for BiCGStab and TFQMR; currently these
   all use the GMRES value, but that value seems off in general (see
   Alan's email about why it works for GMRES).  Test what happens if
   we use the more 'standard' value.  We may need to add some kind of
   internal check to see if the LS object is actually a SUNSPGMR
   object (perhaps check if LS->ops->solve == SUNLinSolSolve_SPGMR ?)

