debug: do not increment mhe.Nk if updatestate! was not called and direct=true

[franckgaga]

This is a minor debug to ensure that the length of the data windows are not incremented if the user did not called updatestate! on a MovingHorizonEstimator with direct=true. This should not happen on normal usage of the package. This is however useful for benchmarking. Now we can call preparestate! multiple time (e.g. with @btime) and the data windows will be identical at each call.

[franckgaga]

Come to think of it, it would create an inconsistency between direct=true and direct=false for the MHE. Calling updatestate! multiple times with direct=false would always increment Nk, but not calling preparestate! multiple times with direct=true. This is not intuitive at all. Let's close this PR. Anyway, this is only useful for benchmarking.

edit: and the other option of not incrementing Nk with multiple updatestate! calls and direct=false is very bad since updatestate! is meant to be called without preparestate! when direct=false (since preparestate! does nothing in this case)