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b31fd17d05
Small values of kₐ make the iterative solver ill-conditioned. This
change reverts to the constant-acceleration feedforward in that case. It
gives _very_ bad results (hence why we added the iterative solver in the
first place), but it's better than hanging.
```
TEST(ArmFeedforwardTest, CalculateIllConditioned) {
constexpr auto Ks = 0.5_V;
constexpr auto Kv = 20_V / 1_rad_per_s;
constexpr auto Ka = 1e-2_V / 1_rad_per_s_sq;
constexpr auto Kg = 0_V;
frc::ArmFeedforward armFF{Ks, Kg, Kv, Ka};
// Calculate(currentAngle, currentVelocity, nextAngle, dt)
CalculateAndSimulate(armFF, 0_rad, 0_rad_per_s, 2_rad_per_s, 20_ms);
}
```
This produces 1 V and doesn't accelerate the system at all. Using
nextVelocity instead of currentVelocity in the feedforward outputs 41 V
and still only accelerates to 0.4 rad/s of the requested 2 rad/s.
I picked the kₐ cutoff by increasing kₐ until the iterative solver
started converging.
Fixes #7743.