Separate Variables from Parameters
Problem: Currently parameters are stored right after the variables in luh, Q and its related arrays (lQi, lQhi, lQhbnd). This make it difficult to provide vectorized PDE options and make some operations where the parameter have to be skipped less intuitive and slower with SIMD
Solution: Introduce a new array like luh to store the parameters. This array is to be passed to the user functions using parameters (nullptr if no parameters) like luh or Q is. The linear space time predictor with split_ck already do this locally.
Issue: Will change the signatures of some user functions and formulation if they are indeed using parameters.