Fix interpolation with VelocityAveraging

[efaulhaber]

Based on #1037.

When interpolating the initial state or after restarting from files (especially when interpolating from VTK files in postprocessing), velocity averaging is not initialized correctly, which leads to zero wall velocity even if the structure is moving. With this PR, interpolation from VTK files in postprocessing works as intended.

[codecov]

Codecov Report

❌ Patch coverage is 90.80460% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.09%. Comparing base (06db3d9) to head (7b89225).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/general/restart.jl	81.35%	11 Missing ⚠️
.../schemes/boundary/wall_boundary/monaghan_kajtar.jl	0.00%	2 Missing ⚠️
src/general/semidiscretization.jl	95.00%	1 Missing ⚠️
src/schemes/boundary/open_boundary/system.jl	97.29%	1 Missing ⚠️
src/schemes/boundary/wall_boundary/system.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1235      +/-   ##
==========================================
+ Coverage   90.04%   90.09%   +0.05%     
==========================================
  Files         136      137       +1     
  Lines       10596    10753     +157     
==========================================
+ Hits         9541     9688     +147     
- Misses       1055     1065      +10     

Flag	Coverage Δ
total	90.10% <90.80%> (+0.05%)	⬆️
unit	75.19% <90.80%> (+4.41%)	⬆️

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