refactor: Rename AmiVector::getLength and AmiVectorArray::getLength to size

[dweindl]

To be more consistent with STL containers and #887.

[codecov]

Codecov Report

❌ Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.60%. Comparing base (d5492c3) to head (b594fde).
⚠️ Report is 125 commits behind head on main.

Files with missing lines	Patch %	Lines
src/model.cpp	0.00%	1 Missing ⚠️
src/steadystateproblem.cpp	50.00%	1 Missing ⚠️
src/sundials_linsol_wrapper.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2965      +/-   ##
==========================================
- Coverage   77.29%   76.60%   -0.70%     
==========================================
  Files         307      307              
  Lines       19912    19912              
  Branches     1502     1502              
==========================================
- Hits        15391    15253     -138     
- Misses       4508     4646     +138     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.75% <88.46%> (-0.02%)	⬇️
cpp_python	38.30% <73.07%> (ø)
petab	39.36% <57.14%> (ø)
python	70.32% <69.23%> (-0.02%)	⬇️
sbmlsuite-jax	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
include/amici/vector.h	88.88% <100.00%> (ø)
src/rdata.cpp	82.59% <100.00%> (ø)
src/solver.cpp	80.87% <100.00%> (ø)
src/solver_cvodes.cpp	73.37% <100.00%> (+0.30%)	⬆️
src/solver_idas.cpp	37.36% <100.00%> (ø)
src/vector.cpp	90.62% <100.00%> (ø)
src/model.cpp	85.37% <0.00%> (ø)
src/steadystateproblem.cpp	79.56% <50.00%> (ø)
src/sundials_linsol_wrapper.cpp	24.81% <66.66%> (ø)

... and 8 files with indirect coverage changes

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