Implement shift parameter for iEnKS and 4D-Var

[patnr]

An email from DWD requested implementing the shift parameter for 4D-Var.

Indeed, it is missing also from the iEnKS implementation. At the moment, the shift parameter is (implicitly) just 1. In practice it should be possible to set anywhere between 1 to DAW. While shift=1 is the optimal value, it is also the most costly one.

Bonus: Make 4D-Var modular, like the iEnKS implementation.