[Core] Fix vOp operations truncating the target vector when an operand is empty - #6445

While replacing accumulateForce with vOp in #6441, a lot of unit tests started failing. The cause: the vOp_* helpers did vv.resize(vb.size()), so when the external force vector is empty (the usual case), the force vector was resized to 0 and everything accumulated so far was lost.

Changes in MechanicalObject.inl:

• accumulating ops (v += b, v += bk, v = vk + b) no longer shrink v: they
  only grow it if b is larger, and only loop over b's size. Nothing is added
  when b is empty.
• those ops, and v *= k, now use a WriteAccessor instead of a WriteOnlyAccessor,
  since they read v and need it to be up to date if it's dirty.
• ops that overwrite v keep the WriteOnlyAccessor, with a comment saying why.
• loops are bounded by the size of the operands, so we never read past the end.

[with-all-tests]

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