changed: D0 length does not change anymore with mhe.direct

[franckgaga]

For the MHE, the measured disturbance vector $\mathbf{D_0}$ will always include one additional data point compared to the other data windows, both with $p=0$ and $p=1$ cases. The time step of the first data point is $k-N_k$ and the last data point is $k$, for a total of $N_k+1$ data points. I just ignore the first data point with $p=1$.

Before this PR, the case with $p=0$ included one additional data point but not $p=1$, which is overly complex for the user and the maintenance of the code. It will clarify the length of the arguments of the nonlinear custom inequality constraint function for the MHE, which is an upcoming feature.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (0dc72a0) to head (4631d6d).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
- Coverage   98.55%   98.55%   -0.01%     
==========================================
  Files          27       27              
  Lines        5250     5245       -5     
==========================================
- Hits         5174     5169       -5     
  Misses         76       76              

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